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Age of pubertal onset and 2nd to 4th digit ratios: Preliminary findings
Early Human Development 116 (2018) 28–32
Contents lists available at ScienceDirect
Early Human Development journal homepage: www.elsevier.com/locate/earlhumdev
Age of pubertal onset and 2nd to 4th digit ratios: Preliminary findings
MARK
⁎
Diane C. Gooding , Blake H. Chambers Department of Psychology, University of Wisconsin, Madison WI, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: 2D:4D Finger length Prenatal hormone exposure Puberty Androgen Menarche Digit ratio
Background: The second to fourth digit ratio (2D:4D) is used as a marker of prenatal sex hormone exposure. The aim of this study was to evaluate the possible relation between digit ratio and age of pubertal onset. Methods: Participants were 321 men and 202 women aged between 18 and 28 at the time of assessment who self-reported their onset of puberty (i.e., age at first conscious ejaculation or menarche). Participants' right and left digit lengths were measured from hand scans. Results: All groups of males, i.e., early, average, and late pubertal onset, had lower mean 2D:4D than the comparable female groups, regardless of the relative timing of their pubertal onset. Among the males, there was a significant difference in digit ratios between individuals who experienced early, average, and late onset of puberty. In the males, we observed a positive relationship between 2D:4D and age of pubertal onset for both right and left hands. There were no significant differences in 2D:4D among the three groups of female maturers. We observed no significant association between digit ratios and age at menarche. Conclusions: These data lend further support for using 2D:4D as a measure of prenatal androgen exposure.
1. Introduction The ratio of the second digit to fourth digit (2D:4D) of the hand is a sexually dimorphic trait marker of prenatal hormone exposure. The 2D:4D ratio appears as early as the 2nd trimester of gestation and is stable by age two [1–3]. Overall, the ratio between the second and fourth digit is associated with the estimated ratio of prenatal testosterone relative to prenatal estradiol; as the 2D:4D ratio increases, the estimated ratio of testosterone to estradiol decreases [4,5]. Various lines of evidence link prenatal hormone levels and digit ratios: genital development and digit development are both controlled by the HoxA and HoxD genes [6]; 2D:4D ratios correlate with prenatal sex steroid levels [3,4]; and smaller digit ratios are associated with a marker of greater androgen sensitivity, namely, androgen receptor alleles with fewer CAG repeats [7,8]; but see [9]. Indeed, using a mouse model, Zheng and Cohn [10] provide experimental evidence that the sexually dimorphic 2D:4D ratio is caused by relative levels of androgen to estrogen signaling in utero. Additional support for an association between androgen sensitivity and 2D:4D comes from studies of children with congenital adrenal hyperplasia (CAH), a condition characterized by very high prenatal androgen level as well as studies of males with Klinefelter's syndrome (KS). Most studies of male and female children with congenital adrenal hyperplasia (CAH) report smaller, more “masculinized” digit ratios than age-matched controls (see, for example, [11,12]). KS males have shown digit ratios similar to those ⁎
displayed by female populations [13]. In typically developing humans, the 2D:4D is significantly higher in females than in males [3,14], whether measured directly in vivo [15] or indirectly from hand photocopies [16]. The sex difference in digit ratios is significantly larger in the right hand than in the left hand [17,18]. Both cross-sectional [3] and longitudinal data [19,20] suggest that sex differences in 2D:4D are unaffected by pubertal onset. Moreover, a recent study [16] of 1656 adults demonstrated that 2D:4D is not strongly associated with adult hormone concentrations. However, to date, relatively few investigators have examined whether there was an association between digit ratios and age of onset of puberty. Thus far, all the investigations have focused on the association between digit ratios and timing of pubertal onset (menarche) in females. To date, there have been three findings of a significant association between 2D:4D and age at menarche. Matchock [21] found evidence for a negative relationship between right 2D:4D and age at menarche, with lower, more masculinized digit ratios being observed among females who reported a later age of menarche. Using a different methodology, Manning and Fink [22] also observed a significant and negative association between right 2D:4D and age at menarche. A later group [23] observed a significantly negative relationship between digit ratios and age at menarche in both left and right hands. It is noteworthy that for all three of the studies that yielded positive findings, the females represented a large age range, typically from 18 through 70 years old. In contrast, the studies finding no significant relationship between either
Corresponding author at: 1202 W. Johnson Street, Madison, WI 53706, USA. E-mail address: [email protected] (D.C. Gooding).
http://dx.doi.org/10.1016/j.earlhumdev.2017.10.007 Received 11 September 2017; Received in revised form 23 October 2017; Accepted 30 October 2017 0378-3782/ © 2017 Elsevier B.V. All rights reserved.
Early Human Development 116 (2018) 28–32
D.C. Gooding, B.H. Chambers
2.4. Age of pubertal onset
left or right hand digit ratio and age at menarche both drew from older samples of females; the Helle [24] investigation focused on females in their post-reproductive stages and the Muller et al. [25] investigation examined females between the ages of 47 and 62. For example, Helle's Finnish sample may not have been representative of contemporary females due to secular trends [22]. Given that food scarcity and other environmental factors may also influence age at menarche, it is possible that cohort effects may have contributed to the negative findings. The purpose of the present investigation was to examine the association between 2D:4D and pubertal onset in the general population. We hoped to reconcile the discrepant findings regarding the relationship between digit ratio and pubertal timing in females, and to extend extant knowledge by investigating the relationship between 2D:4D and age of pubertal onset in males. To our knowledge, this is the first such investigation in males. First, we hypothesized that we would see the well-replicated sex difference in 2D:4D, whereby males displayed smaller digit ratios. Second, we predicted that there would be an association between 2D:4D ratios and age of pubertal onset. Given the evidence linking prenatal hormones, genital development, and digit development, we speculated that amount and duration of intrauterine exposure to hormones, particularly androgens, might affect pubertal onset as well as digit ratios in both males and females. We predicted that 2D:4D would be negatively correlated to age at puberty in females, such that later age at menarche would be associated with lower digit ratios. We also predicted that in males, 2D:4D would be positively correlated to age at pubertal onset. That is, we hypothesized that males with earlier age at puberty would display lower digit ratios relative to the males with later pubertal onset.
All participants were administered a medical history questionnaire as part of their laboratory experience. Embedded in a list of questions regarding their health status, current and past medications and inoculations, participants were queried about their age of pubertal onset. Females were asked to report their onset of menstruation, because the major landmark of puberty for females is menarche. Investigators (such as (27) have used the first conscious ejaculation as a marker of pubertal criterion for males; accordingly, males were asked to report the age at which they experienced this event. We classified participants as being early, average, or late to attain puberty, according to national averages. We used the average age at menarche from a nationally representative sample of U.S. females [28] and Canadian females [29], approximately 12.54 and 12.72 years, respectively. Moreover, the National Longitudinal Survey of Children and Youth [29] operationally defined early maturation as onset before 11.53 years, average maturation as occurring between 11.53 and 13.91 years, and late maturation as onset > 13.91 years. Thus, based on these published norms, we operationally defined early maturation as menarche beginning prior to age 12, and late maturation as menarche occurring after age 14. We defined average maturation as occurring between the ages of 12 and 13 years of age. In our sample, the mean ages of onset for the females in each of the groups are as follows: early (10.68), average (12.47), and late (14.50). The largest cross-sectional study of over 6000 boys [27] indicated a mean age of 13.27 ± 1.08 years as the average age of pubertal onset; this age appeared comparable to other estimates from other countries, despite ethnic and geographic differences. Based on the Tomova et al. findings [27], we operationally defined early maturation as pubertal onset before age 12 and late maturation as pubertal onset after age 14. The mean ages of the males in each of the groups are as follows: early (10.78), average (13.09), and late (15.60). The three groups will be referred to as average maturers, early maturers, and late maturers.
2. Materials and methods 2.1. Participants This was a nonclinical university sample of undergraduates enrolled in an undergraduate Psychology class at a large Midwestern university. None of these individuals had been participants in any prior investigation of digit ratios from this lab [15,26].
3. Results The sample included 321 males and 202 females. Overall, this was a young group; the mean age at the time of assessment was 18.90 ± 1.18 years (range, 18–28). The males did not differ from the females in terms of age at the time of testing, t (521) = 0.81, n.s. Injured or missing digits excluded some measurements, so that scans were not available for all the participants. However, the groups did not differ significantly in terms of missing data. Scans were available for nearly all participants (514 right hands and 519 left hands).1
2.2. Procedure After the experimenters described the study procedures, the participants gave their written informed consent. Ethics approval was obtained from the Educational and Social and Behavioral Sciences Institutional Review Board. Participants received course credit for completion of the study. 2.3. Digit ratios
3.1. Comparison of 2D:4D ratio among average maturers
The assessment of 2D:4D followed the procedure as outlined in [2] and described in [26]. Briefly, each hand was scanned individually using a photo scanner. Participants placed a hand on the glass of the flatbed scanner with fingers lightly touching. A 5 lb weight was used to apply even pressure to all digits to prevent bending, decrease shadows and provide a contrasting background to assist in scoring. Vernier calipers measuring to 0.01 mm were used to measure the distance between the most basal crease on each finger and the most distal point on the fingertip. If the basal crease or the fingertip was not visible, measurements were not included in analyses. Digit ratios were calculated for each hand by dividing the length of the second digit by that of the fourth digit. Digit ratio asymmetry was calculated as the difference between right 2D:4D and left 2D:4D (Dr-l). Two independent raters who were unaware of group membership measured digit lengths. Interrater reliability in digit ratio measurements was calculated using intra-class correlation coefficients. Interrater reliability for the right and left hands was 0.970 and 0.965 respectively.
We first compared the male and female average maturers in terms of their digit ratios. t-Tests for right 2D:4D revealed a significant sex difference, whereby males displayed smaller right digit ratios than females, t (432) = − 6.33, p < 0.001. Similarly, males displayed smaller left digit ratios than the females, t (311) = −6.17, p < 0.001. Table 1 provides digit ratios for the left and right hands for early, average, and late maturers in both males and females. 3.2. Effect of pubertal onset on digit ratios We conducted a two-factor analysis of variance in order to examine the effect of pubertal onset on digit ratios. A 2 (male/female) × 3 (early, average, or late pubertal onset) ANOVA for right digit ratios revealed a significant main effect of sex [F(1, 508) = 33.13, 1 We were missing data from one hand from thirteen participants (11 males and 2 females).
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Early Human Development 116 (2018) 28–32
D.C. Gooding, B.H. Chambers
Table 1 Second to fourth digit ratios. Maturation
Left hand
Right hand
Onset
Males
Females
Males
Females
Early Average Late
0.953 ± 0.030 0.953 ± 0.038 0.967 ± 0.031
0.976 ± 0.042 0.977 ± 0.040 0.971 ± 0.036
0.942 ± 0.029 0.954 ± 0.034 0.967 ± 0.033
0.982 ± 0.033 0.978 ± 0.039 0.971 ± 0.039
Fig. 1. a. Second-to-fourth (2D:4D) digit ratios for male early, average, and late maturers depicted for left and right hands, respectively. Relative maturation determined by self-reported onset of first conscious ejaculation. b. Second-to-fourth (2D:4D) digit ratios for females grouped according to onset of menarche. The early, average, and late maturers' digit ratios are depicted for left and right hands, respectively.
of 1.79 years earlier than the average maturers, whereas those in the late onset group experienced pubertal onset an average of 2.03 years later. However, the three groups did not differ in terms of their right digit ratios, F(2, 198) = 0.77, n.s. (Fig. 1). A 2 (male/female) × 3 (early, average, or late pubertal onset) ANOVA for left digit ratios revealed a significant main effect of sex [F(1, 513) = 17.39, p < 0.001], and a significant puberty timing × sex interaction effect [F(2, 513) = 3.49, p < 0.05]. There was no main effect of puberty timing [F(2,513) = 0.30, n.s.]. Planned contrasts revealed a significant difference in terms of left digit ratios among the males, F(2315) = 3.25, p = 0.04. Males in the late onset group had significantly higher 2D:4D values than males in the average maturers
p < 0.001], and a significant puberty timing × sex interaction effect [F(2, 508) = 4.79, p < 0.01]. There was no main effect of pubertal onset [F(2,508) = 0.71, p = n.s.]. Planned contrasts revealed a significant difference in right digit ratios among the three groups of males, F(2310) = 5.82, p < 0.01. Males in the early group experienced pubertal onset an average of 2.31 years earlier and males in the late group experienced pubertal onset an average of 2.51 years later than the average maturers. Among the males, the early onset group differed significantly from the late onset group in terms of their right 2D:4D, t (df = 3.40), p < 0.01. The average maturers also differed from the late maturers in terms of their right digit ratios, t (df = 310), p < 0.05. Females in the early onset group experienced pubertal onset an average
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Early Human Development 116 (2018) 28–32
D.C. Gooding, B.H. Chambers
Similar to all the investigations to date, age of onset of pubertal events was obtained retrospectively and was not confirmed by medical records. However, pubertal onset is a major developmental event, one that is rather salient in most college students' recent personal histories. Therefore we feel confident in the veracity and accuracy of the retrospective data collected in this investigation. One of the strengths of the present study is that the sample was sufficiently young so that the age of pubertal onset was a not so distal event, increasing the likelihood of accurate recollection and self-report, as well as minimizing the likelihood of cohort effects which may have contributed to earlier null findings. However, the equally compelling findings of Muller et al. [25] and Kalichman et al. [23], which yielded negative and positive findings, respectively, suggest that the issue regarding the relationship between 2D:4D and age at menarche is far from resolved. Further research is required to better characterize the relationship between prenatal sex hormone exposure and 2D:4D measures in females, with greater attention focused on sufficient sample size, methodology, and secular trends. Unlike past research, we observed no difference between the right and left hands in terms of the extent of sexual dimorphism. Furthermore, neither males nor females showed differences in digit ratio asymmetry among the three pubertal onset groups. One caveat is that the present investigation was exploratory in nature. A stronger test of the study hypothesis would require replication with a larger sample of females, especially those with atypical (early and late) pubertal onset. It would also be helpful to repeat these assessments with Tanner staging and/or other objective measures of pubertal maturation. The present study is not without limitations. First, we used an indirect method of measuring 2D:4D. Ribeiro et al. [30] observed that direct measurement of digit ratio tends to result in higher values than indirect measurement. Thus, in terms of our comparison of males and females, our measurement of finger lengths from scans may have resulted in a somewhat larger effect size. Although the magnitude of the effect size for sex differences in 2D:4D is somewhat dependent on (direct versus indirect) measurement protocol, it does not seem likely that the within-sex comparison of maturers is affected, given that all three groups (e.g., early, average, and late) of maturers were measured in the same manner.2 Another limitation of our investigation is that we did not query participants regarding sexual orientation. We did not have approval from our Ethics Board to solicit that sensitive information. We note that diversity in sexual orientation in relation to digit ratios and age of pubertal onset would be of interest for future research inquiry. In conclusion, results from the present study add to the literature supporting the validity of 2D:4D as a tool to study the effects of prenatal androgenization [31]. While our data indicate that digit ratios and pubertal onset are related, further research is needed to explore the precise mechanisms through which they are both affected.
(p < 0.05) and the early maturers (p < 0.01). There were no other significant differences. Among the three groups of female maturers, there was no significant difference in terms of their left digit ratios, F(2, 198) = 0.83, n.s. 3.3. Pubertal onset and digit ratio asymmetry The digit ratios for the two hands were highly correlated, r = 0.62, p < 0.001 for the entire sample. We were interested in whether individuals who differed in terms of the timing of their pubertal onset would differ significantly in terms of the magnitude of their digit ratio asymmetry. The early, average, and late male maturers did not differ significantly in term of their digit ratio asymmetry, F(2, 307) = 0.16, n.s. Similarly, the three groups of female maturers did not differ significantly in terms of their digit ratio asymmetry, F(2, 197) = 0.11, n.s. 4. Discussion and conclusions The goal of the present investigation was to examine whether there is an association between digit ratios and age of onset to puberty. As expected, among individuals who attained puberty within the average time period, males showed smaller digit ratios than females. Indeed, all male participants displayed smaller 2D:4D than the females, regardless of timing of pubertal onset. This finding is therefore consistent with our a priori hypothesis and assertions that digit ratio is a sexually dimorphic trait [3]. The National Longitudinal Survey of Children and Youth [29] operationally defined early maturation as an onset < 11.53 years, average maturation as occurring between 11.53 and 13.91 years and late maturation as onset > 13.91 years. Among both males and females, the three groups of maturers in our study fell well within the maturation groups operationalized by the National Longitudinal Survey. The groups in the present investigation did not differ in terms of age at time of assessment, though they differed in terms of timing of pubertal onset. The most novel finding was that in males, digit ratios were positively correlated to age at pubertal onset. That is, compared to males who reportedly achieved puberty at the average time, the early onset group showed lower, hypermasculinized ratios and the late onset group showed higher, more feminized ratios in the right hand. We also found the positive relationship between age of pubertal onset and second to fourth digit ratio in the left hand, with the late maturers displaying significantly higher, more feminized ratios than either the early maturers or average maturers. Contrary to our predictions, the female participants' digit ratios did not differ significantly as a function of pubertal onset. This null finding was unexpected, given that the females in the present investigation were most similar to those in the Matchock [21] sample, i.e., American college students, predominantly Caucasian, and a mean menarcheal age of 13.05 ± 1.26 years (compared to our sample mean age of 12.75 ± 1.30). Matchock calculated a correlation between menarcheal age and digit ratios, whereas we chose to compare the women's' mean digit ratios after dividing them according to menarcheal age. When we attempted to replicate their methods by calculating a Spearman's rho, we also observed a negative association between right digit ratio and age at menarche, though it failed to reach statistical significance. While the proportion of women in the Matchock [21] sample who experienced menarche relatively early or late is unclear, it is possible that the differential findings may be attributable to sample differences and differences in statistical power to detect effects. This is plausible, because although Manning et al. [22] also observed a negative and significant relationship between digit ratios and age at menarche, the investigators found a smaller effect size than the earlier Matchock [21] study. The smaller effect size in the Manning et al. investigation was noteworthy, given that the study was based upon a sample of nearly 71,000 Caucasian women.
Conflicts of interest None. Acknowledgements We gratefully acknowledge all of our study participants. We would also like to thank the following: Danny Cubberly, Kevin Clancy, and S. Bevlin Jennings for performing the digit measurements; S. Bevlin Jennings and Madeline Johnson Pflum for assistance with data collection. This research was supported by a grant to DCG from the Vilas 2 Prior research has relied upon direct, indirect (photocopies or scans), or radiological measurements and subsequent visual classification of finger length ratio (see Kalichman et al. [23]). Although significant relationships have been observed between 2D:4D and pubertal timing using a range of methods, future work in this area would benefit from using direct finger measurement where possible.
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152–160. [16] D.C. Muller, G.G. Giles, J. Bassett, H.A. Morris, J.T. Manning, J.L. Hopper, et al., Second to fourth digit ratio (2D:4D) and concentrations of circulating sex hormones in adulthood, Reprod. Biol. Endocrinol. 9 (2011) 57. [17] J.T. Manning, B. Fink, Digit ratio (2D:4D), dominance, reproductive success, asymmetry, and sociosexuality in the BBC internet study, Am. J. Hum. Biol. 20 (2008) 451–461. [18] J. Hönekopp, S. Watson, Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand, Am. J. Hum. Biol. 22 (2010) 619–630. [19] M.H. McIntyre, P.T. Ellison, D.E. Lieberman, E. Demerath, B. Towne, The development of sex differences in digit formula from infancy in the Fels longitudinal study, Proc. Biol. Sci. 272 (1571) (2005) 1473–1479. [20] R. Drivers, J. Manning, A. Jacobson, A longitudinal study of digit ratio (2D:4D) and other finger ratios in Jamaican children, Horm. Behav. 49 (2) (2006) 150–156. [21] R.L. Matchock, Low digit ratio (2D:4D) is associated with delayed menarche, Am. J. Hum. Biol. 20 (2008) 487–489. [22] J.T. Manning, B. Fink, Is low digit ratio linked with late menarche? Evidence from the BBC internet study, Am. J. Hum. Biol. 23 (2011) 527–533. [23] L. Kalichman, V. Batsevich, E. Kobyliansky, 2D:4D finger length ratio and reproductive indices in a Chuvashian population, Am. J. Hum. Biol. 25 (2013) 617–621. [24] S. Helle, Does second-to-fourth digit length ratio (2D:4d) predict age at menarche in women? Am. J. Hum. Biol. 22 (2010) 418–420. [25] D.C. Muller, L. Baglietto, J.T. Manning, C. M, J.L. Hopper, D.R. English, et al., Second to fourth digit ratio (2D:4D), breast cancer risk factors, and breast cancer risk: a prospective cohort study, Br. J. Cancer 107 (2012) 1631–1636. [26] D.C. Gooding, M. Johnson, J.S. Peterman, Schizotypy and altered digit ratios: a second look, Psychiatry Res. 178 (1) (2010) 73–78. [27] A. Tomova, C. Lalabonova, R.N. Robeva, P.T. Kumanova, Timing of pubertal maturation according to the age at first conscious ejaculation, Andrologia 43 (2011) 163–166. [28] S.E. Anderson, G.E. Dallal, A. Must, Relative weight and race influence average age at menarche: results from two nationally representative surveys of US girls studied 25 years apart, Pediatrics 111 (4 Pt. 1) (2003) 844–850. [29] B. Al-Sahab, C.I. Ardern, M.J. Hamadeh, H. Tamim, Age at menarche in Canada: results from the National Longitudinal Survey of children and youth, BMC Public Health 10 (2010). [30] E. Ribeiro, N. Neave, R.N. Morais, J.T. Manning, Direct versus indirect measurement of digit ratio (2D:4D): a critical review of the literature and new data, Evol. Psychol. (2016) 1–8. [31] J. Hönekopp, L. Bartholdt, L. Beier, A. Liebert, Second to fourth digit length ratio (2D:4D) and adult sex hormone levels: new data and a meta-analytic review, Psychoneuroendocrinology 32 (2007) 313–321.
Foundation. We thank the anonymous reviewers for their invaluable feedback regarding an earlier draft of our manuscript. References [1] M.A. Malas, S. Dogan, E.H. Evcil, K. Desdicioglu, Fetal development of the hand, digits, and digit ratio, Early Hum. Dev. 82 (2006) 469–475. [2] J.T. Manning, Digit Ratio: A Pointer to Fertility, Behavior, and Health, Rutgers University Press, New Brunswick, New Jersey, 2002. [3] J.T. Manning, D. Scutt, J. Wilson, D.I. Lewis-Jones, The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen, Hum. Reprod. 113 (1998) 3000–3004. [4] S. Lutchmaya, S. Baron-Cohen, P. Raggatt, R. Knickmeyer, J.T. Manning, 2nd to 4th digit ratios, fetal testosterone and estradiol, Early Hum. Dev. 77 (2004) 23–28. [5] T. Ventura, M.C. Gomes, A. Pita, M.T. Neto, A. Taylor, Digit ratio (2D:4D) in newborns: influences of prenatal testosterone and maternal environment, Early Hum. Dev. 89 (2013) 107–112. [6] T. Kondo, J. Zákány, J.W. Innis, D. Duboule, Of fingers, toes, and penises, Nature 390 (6655) (1997) 29. [7] J.T. Manning, P.E. Bundred, D.J. Newton, B.F. Flanagan, The second to fourth digit ratio and variation in the androgen receptor gene, Evol. Hum. Behav. 24 (2003) 399–405. [8] M. Evardone, G.M. Alexander, Anxiety, sex-linked behaviors, and digit ratios (2D: 4D), Arch. Sex. Behav. 38 (2009) 442–455. [9] P.L. Hurd, K.L. Vaillancourt, N.L. Dinsdale, Aggression, digit ratio and variation in androgen receptor and monoamine oxidase a genes in men, Behav. Genet. 41 (2011) 543–556. [10] Z. Zheng, M.J. Cohn, Developmental basis of sexually dimorphic digit ratios, Proc. Natl. Acad. Sci. U. S. A. 108 (39) (2011) 16289–16294. [11] W.M. Brown, M. Hines, B.A. Fane, S.M. Breedlove, Masculinized finger length patterns in human males and females with congenital adrenal hyperplasia, Horm. Behav. 42 (2002) 380–386. [12] A. Ökten, M. Kalyoncu, N. Yaris, The ratio of second- and fourth-digit lengths and congential adrenal hyperplasia due to 21-hydroxylase deficiency, Early Hum. Dev. 70 (2002) 47–54. [13] J.T. Manning, L.P. Kilduff, R. Trivers, Digit ratio (2D:4D) in Klinefelter's syndrome, Andrology 1 (2013) 94–99. [14] J.T. Manning, L. Barley, J. Walton, D.I. Lewis-Jones, R.L. Trivers, D. Singh, et al., The 2nd to 4th digit ratio, sexual dimorphism, population differences, and reproductive success: evidence for sexually antagonistic genes? Evol. Hum. Behav. 21 (2000) 163–183. [15] M.P. Daly, D.C. Gooding, H.M. Jessen, A.P. Auger, Indicators of developmental deviance in individuals at risk for schizophrenia, Schizophr. Res. 101 (1–3) (2008)
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Contents lists available at ScienceDirect
Early Human Development journal homepage: www.elsevier.com/locate/earlhumdev
Age of pubertal onset and 2nd to 4th digit ratios: Preliminary findings
MARK
⁎
Diane C. Gooding , Blake H. Chambers Department of Psychology, University of Wisconsin, Madison WI, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: 2D:4D Finger length Prenatal hormone exposure Puberty Androgen Menarche Digit ratio
Background: The second to fourth digit ratio (2D:4D) is used as a marker of prenatal sex hormone exposure. The aim of this study was to evaluate the possible relation between digit ratio and age of pubertal onset. Methods: Participants were 321 men and 202 women aged between 18 and 28 at the time of assessment who self-reported their onset of puberty (i.e., age at first conscious ejaculation or menarche). Participants' right and left digit lengths were measured from hand scans. Results: All groups of males, i.e., early, average, and late pubertal onset, had lower mean 2D:4D than the comparable female groups, regardless of the relative timing of their pubertal onset. Among the males, there was a significant difference in digit ratios between individuals who experienced early, average, and late onset of puberty. In the males, we observed a positive relationship between 2D:4D and age of pubertal onset for both right and left hands. There were no significant differences in 2D:4D among the three groups of female maturers. We observed no significant association between digit ratios and age at menarche. Conclusions: These data lend further support for using 2D:4D as a measure of prenatal androgen exposure.
1. Introduction The ratio of the second digit to fourth digit (2D:4D) of the hand is a sexually dimorphic trait marker of prenatal hormone exposure. The 2D:4D ratio appears as early as the 2nd trimester of gestation and is stable by age two [1–3]. Overall, the ratio between the second and fourth digit is associated with the estimated ratio of prenatal testosterone relative to prenatal estradiol; as the 2D:4D ratio increases, the estimated ratio of testosterone to estradiol decreases [4,5]. Various lines of evidence link prenatal hormone levels and digit ratios: genital development and digit development are both controlled by the HoxA and HoxD genes [6]; 2D:4D ratios correlate with prenatal sex steroid levels [3,4]; and smaller digit ratios are associated with a marker of greater androgen sensitivity, namely, androgen receptor alleles with fewer CAG repeats [7,8]; but see [9]. Indeed, using a mouse model, Zheng and Cohn [10] provide experimental evidence that the sexually dimorphic 2D:4D ratio is caused by relative levels of androgen to estrogen signaling in utero. Additional support for an association between androgen sensitivity and 2D:4D comes from studies of children with congenital adrenal hyperplasia (CAH), a condition characterized by very high prenatal androgen level as well as studies of males with Klinefelter's syndrome (KS). Most studies of male and female children with congenital adrenal hyperplasia (CAH) report smaller, more “masculinized” digit ratios than age-matched controls (see, for example, [11,12]). KS males have shown digit ratios similar to those ⁎
displayed by female populations [13]. In typically developing humans, the 2D:4D is significantly higher in females than in males [3,14], whether measured directly in vivo [15] or indirectly from hand photocopies [16]. The sex difference in digit ratios is significantly larger in the right hand than in the left hand [17,18]. Both cross-sectional [3] and longitudinal data [19,20] suggest that sex differences in 2D:4D are unaffected by pubertal onset. Moreover, a recent study [16] of 1656 adults demonstrated that 2D:4D is not strongly associated with adult hormone concentrations. However, to date, relatively few investigators have examined whether there was an association between digit ratios and age of onset of puberty. Thus far, all the investigations have focused on the association between digit ratios and timing of pubertal onset (menarche) in females. To date, there have been three findings of a significant association between 2D:4D and age at menarche. Matchock [21] found evidence for a negative relationship between right 2D:4D and age at menarche, with lower, more masculinized digit ratios being observed among females who reported a later age of menarche. Using a different methodology, Manning and Fink [22] also observed a significant and negative association between right 2D:4D and age at menarche. A later group [23] observed a significantly negative relationship between digit ratios and age at menarche in both left and right hands. It is noteworthy that for all three of the studies that yielded positive findings, the females represented a large age range, typically from 18 through 70 years old. In contrast, the studies finding no significant relationship between either
Corresponding author at: 1202 W. Johnson Street, Madison, WI 53706, USA. E-mail address: [email protected] (D.C. Gooding).
http://dx.doi.org/10.1016/j.earlhumdev.2017.10.007 Received 11 September 2017; Received in revised form 23 October 2017; Accepted 30 October 2017 0378-3782/ © 2017 Elsevier B.V. All rights reserved.
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2.4. Age of pubertal onset
left or right hand digit ratio and age at menarche both drew from older samples of females; the Helle [24] investigation focused on females in their post-reproductive stages and the Muller et al. [25] investigation examined females between the ages of 47 and 62. For example, Helle's Finnish sample may not have been representative of contemporary females due to secular trends [22]. Given that food scarcity and other environmental factors may also influence age at menarche, it is possible that cohort effects may have contributed to the negative findings. The purpose of the present investigation was to examine the association between 2D:4D and pubertal onset in the general population. We hoped to reconcile the discrepant findings regarding the relationship between digit ratio and pubertal timing in females, and to extend extant knowledge by investigating the relationship between 2D:4D and age of pubertal onset in males. To our knowledge, this is the first such investigation in males. First, we hypothesized that we would see the well-replicated sex difference in 2D:4D, whereby males displayed smaller digit ratios. Second, we predicted that there would be an association between 2D:4D ratios and age of pubertal onset. Given the evidence linking prenatal hormones, genital development, and digit development, we speculated that amount and duration of intrauterine exposure to hormones, particularly androgens, might affect pubertal onset as well as digit ratios in both males and females. We predicted that 2D:4D would be negatively correlated to age at puberty in females, such that later age at menarche would be associated with lower digit ratios. We also predicted that in males, 2D:4D would be positively correlated to age at pubertal onset. That is, we hypothesized that males with earlier age at puberty would display lower digit ratios relative to the males with later pubertal onset.
All participants were administered a medical history questionnaire as part of their laboratory experience. Embedded in a list of questions regarding their health status, current and past medications and inoculations, participants were queried about their age of pubertal onset. Females were asked to report their onset of menstruation, because the major landmark of puberty for females is menarche. Investigators (such as (27) have used the first conscious ejaculation as a marker of pubertal criterion for males; accordingly, males were asked to report the age at which they experienced this event. We classified participants as being early, average, or late to attain puberty, according to national averages. We used the average age at menarche from a nationally representative sample of U.S. females [28] and Canadian females [29], approximately 12.54 and 12.72 years, respectively. Moreover, the National Longitudinal Survey of Children and Youth [29] operationally defined early maturation as onset before 11.53 years, average maturation as occurring between 11.53 and 13.91 years, and late maturation as onset > 13.91 years. Thus, based on these published norms, we operationally defined early maturation as menarche beginning prior to age 12, and late maturation as menarche occurring after age 14. We defined average maturation as occurring between the ages of 12 and 13 years of age. In our sample, the mean ages of onset for the females in each of the groups are as follows: early (10.68), average (12.47), and late (14.50). The largest cross-sectional study of over 6000 boys [27] indicated a mean age of 13.27 ± 1.08 years as the average age of pubertal onset; this age appeared comparable to other estimates from other countries, despite ethnic and geographic differences. Based on the Tomova et al. findings [27], we operationally defined early maturation as pubertal onset before age 12 and late maturation as pubertal onset after age 14. The mean ages of the males in each of the groups are as follows: early (10.78), average (13.09), and late (15.60). The three groups will be referred to as average maturers, early maturers, and late maturers.
2. Materials and methods 2.1. Participants This was a nonclinical university sample of undergraduates enrolled in an undergraduate Psychology class at a large Midwestern university. None of these individuals had been participants in any prior investigation of digit ratios from this lab [15,26].
3. Results The sample included 321 males and 202 females. Overall, this was a young group; the mean age at the time of assessment was 18.90 ± 1.18 years (range, 18–28). The males did not differ from the females in terms of age at the time of testing, t (521) = 0.81, n.s. Injured or missing digits excluded some measurements, so that scans were not available for all the participants. However, the groups did not differ significantly in terms of missing data. Scans were available for nearly all participants (514 right hands and 519 left hands).1
2.2. Procedure After the experimenters described the study procedures, the participants gave their written informed consent. Ethics approval was obtained from the Educational and Social and Behavioral Sciences Institutional Review Board. Participants received course credit for completion of the study. 2.3. Digit ratios
3.1. Comparison of 2D:4D ratio among average maturers
The assessment of 2D:4D followed the procedure as outlined in [2] and described in [26]. Briefly, each hand was scanned individually using a photo scanner. Participants placed a hand on the glass of the flatbed scanner with fingers lightly touching. A 5 lb weight was used to apply even pressure to all digits to prevent bending, decrease shadows and provide a contrasting background to assist in scoring. Vernier calipers measuring to 0.01 mm were used to measure the distance between the most basal crease on each finger and the most distal point on the fingertip. If the basal crease or the fingertip was not visible, measurements were not included in analyses. Digit ratios were calculated for each hand by dividing the length of the second digit by that of the fourth digit. Digit ratio asymmetry was calculated as the difference between right 2D:4D and left 2D:4D (Dr-l). Two independent raters who were unaware of group membership measured digit lengths. Interrater reliability in digit ratio measurements was calculated using intra-class correlation coefficients. Interrater reliability for the right and left hands was 0.970 and 0.965 respectively.
We first compared the male and female average maturers in terms of their digit ratios. t-Tests for right 2D:4D revealed a significant sex difference, whereby males displayed smaller right digit ratios than females, t (432) = − 6.33, p < 0.001. Similarly, males displayed smaller left digit ratios than the females, t (311) = −6.17, p < 0.001. Table 1 provides digit ratios for the left and right hands for early, average, and late maturers in both males and females. 3.2. Effect of pubertal onset on digit ratios We conducted a two-factor analysis of variance in order to examine the effect of pubertal onset on digit ratios. A 2 (male/female) × 3 (early, average, or late pubertal onset) ANOVA for right digit ratios revealed a significant main effect of sex [F(1, 508) = 33.13, 1 We were missing data from one hand from thirteen participants (11 males and 2 females).
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Table 1 Second to fourth digit ratios. Maturation
Left hand
Right hand
Onset
Males
Females
Males
Females
Early Average Late
0.953 ± 0.030 0.953 ± 0.038 0.967 ± 0.031
0.976 ± 0.042 0.977 ± 0.040 0.971 ± 0.036
0.942 ± 0.029 0.954 ± 0.034 0.967 ± 0.033
0.982 ± 0.033 0.978 ± 0.039 0.971 ± 0.039
Fig. 1. a. Second-to-fourth (2D:4D) digit ratios for male early, average, and late maturers depicted for left and right hands, respectively. Relative maturation determined by self-reported onset of first conscious ejaculation. b. Second-to-fourth (2D:4D) digit ratios for females grouped according to onset of menarche. The early, average, and late maturers' digit ratios are depicted for left and right hands, respectively.
of 1.79 years earlier than the average maturers, whereas those in the late onset group experienced pubertal onset an average of 2.03 years later. However, the three groups did not differ in terms of their right digit ratios, F(2, 198) = 0.77, n.s. (Fig. 1). A 2 (male/female) × 3 (early, average, or late pubertal onset) ANOVA for left digit ratios revealed a significant main effect of sex [F(1, 513) = 17.39, p < 0.001], and a significant puberty timing × sex interaction effect [F(2, 513) = 3.49, p < 0.05]. There was no main effect of puberty timing [F(2,513) = 0.30, n.s.]. Planned contrasts revealed a significant difference in terms of left digit ratios among the males, F(2315) = 3.25, p = 0.04. Males in the late onset group had significantly higher 2D:4D values than males in the average maturers
p < 0.001], and a significant puberty timing × sex interaction effect [F(2, 508) = 4.79, p < 0.01]. There was no main effect of pubertal onset [F(2,508) = 0.71, p = n.s.]. Planned contrasts revealed a significant difference in right digit ratios among the three groups of males, F(2310) = 5.82, p < 0.01. Males in the early group experienced pubertal onset an average of 2.31 years earlier and males in the late group experienced pubertal onset an average of 2.51 years later than the average maturers. Among the males, the early onset group differed significantly from the late onset group in terms of their right 2D:4D, t (df = 3.40), p < 0.01. The average maturers also differed from the late maturers in terms of their right digit ratios, t (df = 310), p < 0.05. Females in the early onset group experienced pubertal onset an average
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Similar to all the investigations to date, age of onset of pubertal events was obtained retrospectively and was not confirmed by medical records. However, pubertal onset is a major developmental event, one that is rather salient in most college students' recent personal histories. Therefore we feel confident in the veracity and accuracy of the retrospective data collected in this investigation. One of the strengths of the present study is that the sample was sufficiently young so that the age of pubertal onset was a not so distal event, increasing the likelihood of accurate recollection and self-report, as well as minimizing the likelihood of cohort effects which may have contributed to earlier null findings. However, the equally compelling findings of Muller et al. [25] and Kalichman et al. [23], which yielded negative and positive findings, respectively, suggest that the issue regarding the relationship between 2D:4D and age at menarche is far from resolved. Further research is required to better characterize the relationship between prenatal sex hormone exposure and 2D:4D measures in females, with greater attention focused on sufficient sample size, methodology, and secular trends. Unlike past research, we observed no difference between the right and left hands in terms of the extent of sexual dimorphism. Furthermore, neither males nor females showed differences in digit ratio asymmetry among the three pubertal onset groups. One caveat is that the present investigation was exploratory in nature. A stronger test of the study hypothesis would require replication with a larger sample of females, especially those with atypical (early and late) pubertal onset. It would also be helpful to repeat these assessments with Tanner staging and/or other objective measures of pubertal maturation. The present study is not without limitations. First, we used an indirect method of measuring 2D:4D. Ribeiro et al. [30] observed that direct measurement of digit ratio tends to result in higher values than indirect measurement. Thus, in terms of our comparison of males and females, our measurement of finger lengths from scans may have resulted in a somewhat larger effect size. Although the magnitude of the effect size for sex differences in 2D:4D is somewhat dependent on (direct versus indirect) measurement protocol, it does not seem likely that the within-sex comparison of maturers is affected, given that all three groups (e.g., early, average, and late) of maturers were measured in the same manner.2 Another limitation of our investigation is that we did not query participants regarding sexual orientation. We did not have approval from our Ethics Board to solicit that sensitive information. We note that diversity in sexual orientation in relation to digit ratios and age of pubertal onset would be of interest for future research inquiry. In conclusion, results from the present study add to the literature supporting the validity of 2D:4D as a tool to study the effects of prenatal androgenization [31]. While our data indicate that digit ratios and pubertal onset are related, further research is needed to explore the precise mechanisms through which they are both affected.
(p < 0.05) and the early maturers (p < 0.01). There were no other significant differences. Among the three groups of female maturers, there was no significant difference in terms of their left digit ratios, F(2, 198) = 0.83, n.s. 3.3. Pubertal onset and digit ratio asymmetry The digit ratios for the two hands were highly correlated, r = 0.62, p < 0.001 for the entire sample. We were interested in whether individuals who differed in terms of the timing of their pubertal onset would differ significantly in terms of the magnitude of their digit ratio asymmetry. The early, average, and late male maturers did not differ significantly in term of their digit ratio asymmetry, F(2, 307) = 0.16, n.s. Similarly, the three groups of female maturers did not differ significantly in terms of their digit ratio asymmetry, F(2, 197) = 0.11, n.s. 4. Discussion and conclusions The goal of the present investigation was to examine whether there is an association between digit ratios and age of onset to puberty. As expected, among individuals who attained puberty within the average time period, males showed smaller digit ratios than females. Indeed, all male participants displayed smaller 2D:4D than the females, regardless of timing of pubertal onset. This finding is therefore consistent with our a priori hypothesis and assertions that digit ratio is a sexually dimorphic trait [3]. The National Longitudinal Survey of Children and Youth [29] operationally defined early maturation as an onset < 11.53 years, average maturation as occurring between 11.53 and 13.91 years and late maturation as onset > 13.91 years. Among both males and females, the three groups of maturers in our study fell well within the maturation groups operationalized by the National Longitudinal Survey. The groups in the present investigation did not differ in terms of age at time of assessment, though they differed in terms of timing of pubertal onset. The most novel finding was that in males, digit ratios were positively correlated to age at pubertal onset. That is, compared to males who reportedly achieved puberty at the average time, the early onset group showed lower, hypermasculinized ratios and the late onset group showed higher, more feminized ratios in the right hand. We also found the positive relationship between age of pubertal onset and second to fourth digit ratio in the left hand, with the late maturers displaying significantly higher, more feminized ratios than either the early maturers or average maturers. Contrary to our predictions, the female participants' digit ratios did not differ significantly as a function of pubertal onset. This null finding was unexpected, given that the females in the present investigation were most similar to those in the Matchock [21] sample, i.e., American college students, predominantly Caucasian, and a mean menarcheal age of 13.05 ± 1.26 years (compared to our sample mean age of 12.75 ± 1.30). Matchock calculated a correlation between menarcheal age and digit ratios, whereas we chose to compare the women's' mean digit ratios after dividing them according to menarcheal age. When we attempted to replicate their methods by calculating a Spearman's rho, we also observed a negative association between right digit ratio and age at menarche, though it failed to reach statistical significance. While the proportion of women in the Matchock [21] sample who experienced menarche relatively early or late is unclear, it is possible that the differential findings may be attributable to sample differences and differences in statistical power to detect effects. This is plausible, because although Manning et al. [22] also observed a negative and significant relationship between digit ratios and age at menarche, the investigators found a smaller effect size than the earlier Matchock [21] study. The smaller effect size in the Manning et al. investigation was noteworthy, given that the study was based upon a sample of nearly 71,000 Caucasian women.
Conflicts of interest None. Acknowledgements We gratefully acknowledge all of our study participants. We would also like to thank the following: Danny Cubberly, Kevin Clancy, and S. Bevlin Jennings for performing the digit measurements; S. Bevlin Jennings and Madeline Johnson Pflum for assistance with data collection. This research was supported by a grant to DCG from the Vilas 2 Prior research has relied upon direct, indirect (photocopies or scans), or radiological measurements and subsequent visual classification of finger length ratio (see Kalichman et al. [23]). Although significant relationships have been observed between 2D:4D and pubertal timing using a range of methods, future work in this area would benefit from using direct finger measurement where possible.
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