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Born to be a Marine: Digit ratios and military service
Personality and Individual Differences 53 (2012) 166–168
Contents lists available at SciVerse ScienceDirect
Personality and Individual Differences journal homepage: www.elsevier.com/locate/paid
Born to be a Marine: Digit ratios and military service HaengRyang Huh ⇑ Department of Communication Arts, Sejong University, Chiphyun Bldg 513, 98 Kunja-Dong, Kwangjin-Ku, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 January 2012 Received in revised form 8 February 2012 Accepted 28 February 2012 Available online 25 March 2012 Keywords: Digit ratio Risk-taking Testosterone Occupations Military service Marines Army
a b s t r a c t Digit ratio (2D:4D) is a negative correlate of prenatal testosterone. The current study tested the hypothesis that 2D:4D would be associated with the type of military service chosen by young Korean males. Our sample consisted of 128 young males aged 22–26 years (M = 24.32, SD = 0.89). We measured the 2D:4D on the participants’ right hands and explored its relationship to their selection of one of four branches of military service. Statistical analysis found no significant differences among the four branches of military service but supported our hypothesis of significant group differences in 2D:4D between Marines and all non-Marines. As expected, members of the Marine Corps demonstrated the lowest digit ratios, whereas those in the Army showed the highest. The average 2D:4D was 0.94 (SD = 0.05) for the Marine Corps, 0.95 (SD = 0.07) for the Air Force, 0.96 (SD = 0.06) for the Navy, and 0.97 (SD = 0.06) for the Army. Our research found evidence of small but significant group differences in 2D:4D among members of different branches of military service. We conclude that low 2D:4D is related to the risk and severity associated with the type of military training selected by recruits. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Established studies provide compelling evidence of the close association of occupational choice and achievement with both testosterone level and digit ratio. One previous study demonstrated occupational differences in testosterone concentrations among actors, ministers, firemen, sales people, physicians, football players, and professors (Dabbs, de La Rue, & Williams, 1990). Male and female trial lawyers had testosterone levels that were higher than those of nontrial lawyers of the same sex (Dabbs, Alford, & Fielden, 1998). Individuals with higher testosterone levels and family business backgrounds were more likely to create new business ventures (White, Thornhill, & Hampson, 2007). The literature also provides strong support for hormonal androgen influences on interest in occupations characterized by working with things versus with people (Beltz, Swanson, & Berenbaum, 2011). Likewise, previous studies have demonstrated a significant relationship between occupational choice and digit ratio. Low 2D:4D (the relative lengths of the second and fourth fingers), indicating high levels of prenatal testosterone, was related to a ‘‘masculinized’’ score for career interests in groups of both males and females according to different scales (Weis, Firker, & Hennig, 2007). Significant differences in digit ratios among faculty in science and social science departments have also been demonstrated (Brosnan, 2006). Additionally, significant correlations of finger length (positive) and digit ratio (negative) with realistic interests ⇑ Corresponding author. Tel.: +82 2 3408 3707; fax: +82 2 3408 4307. E-mail addresses: [email protected], [email protected] 0191-8869/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.paid.2012.02.021
and significant correlations between finger length (negative) and social interests, a marker of the people–things dimension, have also been reported (Hell & PaBler, 2011). Manning, Reimers, Baron-Cohen, Wheelwright, and Fink (2010) reported that low 2D:4D was found in individuals in male-dominated occupations; this finding was particularly robust among females. These studies addressed career interests but not aspects of risk taking. Manning, Scutt, Wilson, and Lewis-Jones (1998) suggested that 2D:4D ratio was negatively correlated with prenatal testosterone. Evidence for this hypothesis was subsequently reviewed by Manning (2002, 2008) and Breedlove (2010). Furthermore, recent experimental work on the fetuses of mammals has essentially confirmed that the 2D:4D ratio is negatively associated with prenatal testosterone and positively associated with prenatal estrogen (Manning, 2011; Talarovicova, Krskova, & Blazekova, 2009; Zhengui & Martin, 2011). If digit ratios were taken as an indicator of prenatal exposure to testosterone, the relationship between individual thoughts and behaviors and digit ratios should follow the same pattern as the relationship between these phenomena and testosterone. Indeed, digit ratios offer a valid test of the organizational hypothesis that androgens act early in life to masculinize various human behaviors (Breedlove, 2010). Previous research investigating the relationships between digit ratios (2D:4D) and the propensity of individuals to engage in risky and aggressive behavior has revealed associations between digit ratios and personality traits (Fink, Manning, & Neave, 2004; Lindova, Hruskova, Pivonkova, Kubena, & Flegr, 2008). Digit ratios have also been associated with recreational, financial, and social risk-taking behaviors (Stenstrom,
H. Huh / Personality and Individual Differences 53 (2012) 166–168
Saad, Nepomuceno, & Mendenhall, 2011). Thus, digit ratios have been linked to individuals’ risk taking. Testosterone and risk taking are closely related, and evidence has suggested an association between 2D:4D and risk taking. Korea requires a specific amount of military service from every male citizen. Some young males in Korea volunteer to be members of the Marine Corps, Navy, or Air Force in advance, whereas most males wait to be conscripted by the Army. Military branches vary according to length of service and severity of training. The lengths of service are 21 months for the Army and Marine Corps, 23 months for the Navy, and 24 months for the Air Force (Ministry of National Defense, 2011). The four options for military service involve different degrees of severity in the requisite training. It is commonly believed that the Navy and Air Force require the mildest training, whereas the Marine Corps requires the harshest. Different types of military services also require individuals to engage in different levels of risk depending on length of service and severity of training. Individuals usually differ with regard to level of risk-taking behavior, including military service. Most studies have not focused on the relationship between risk taking and digit ratios. Indeed, one previous study provided no evidence for this perspective, showing instead that 2D:4D in firefighters (N = 134) was not lower than that of controls drawn from the local male population (Voracek, Pum, & Dressler, 2010). However, this study also reported several other interesting findings, including an association between low 2D:4D and low rank and the absence of evidence for a link between low 2D:4D and superior performance in sports. These conclusions are so unexpected that they may indicate that the reported findings are unreliable. Thus, this study explored the relationship between digit ratio and military service. Individuals differ with respect to the propensity to take risks, which suggests that digit ratios may be associated with the probability of volunteering for different branches of military service. Individuals with lower digit ratios may tend to prefer higher-risk military branches than would those with higher digit ratios. We hypothesized that individuals with lower digit ratios would tend to prefer branches with more severe training than would those with higher ratios. The current study was designed to explore the relationship between digit ratios and choices among different branches of military service. 2. Method 2.1. Participants
167
greater risk than would individuals with lower levels of testosterone. First, participants were asked to complete questionnaires that solicited information on demographic variables such as age and type of military service. Before distributing the survey, we used a focus group to explore the degree of risk taking in four types of military service. The focus group was composed of 7 student veterans. This focus group of seven veterans rated the Marine Corps highest on the spectrum of risk taking; the Navy and Air Force followed, and the Army was rated lowest. Second, we photographed participants’ hands to determine digit ratios. We included the traditional 2D:4D measure as a proxy for prenatal androgen exposure. The current study focused on digit ratios in the right hand because this hand tends to be more sex dimorphic (Manning, 2002). Finger lengths were measured from the ventral proximal crease to the fingertip by using Adobe Photoshop ruler. To check for reliability, a second independent rater also measured finger lengths. The inter-rater correlation coefficient for the two measurements of 2D:4D was 0.86 (p < 0.001), which indicated a high level of reliability.
3. Results The average digit ratio for all participants was 0.96 (SD = 0.06). First, we used a one-way analysis of variance to explore group differences in digit ratios among members of the Marine Corps, Navy, Air Force, and Army. As Fig. 1 demonstrates, the average digit ratio was 0.94 (SD = 0.05) for the Marine Corps, 0.95 (SD = 0.07) for the Air Force, 0.96 (SD = 0.06) for the Navy, and 0.97 (SD = 0.06) for the Army. A one-way analysis of variance demonstrated non-significant group differences in digit ratios among members of the four branches of military service [F(2, 125) = 1.822, p = 0.147]. Second, the statistical analysis demonstrated significant group differences in digit ratios between Marines and non-Marines. As illustrated by Fig. 2, the average digit ratio for Marines was 0.94 (SD = 0.05), whereas that for non-Marines was 0.96 (SD = 0.06). Statistical analysis demonstrated significant group differences in digit ratios between Marines and non-Marines [t(127) = 2.33, p = 0.02]. As expected, Marines had lower digit ratios than did non-Marines. We then explored whether significant difference between the Marine Corps and each of the other military services could be observed. First, statistical analysis demonstrated a significant difference in digit ratios between members of the Marine Corps and members of the Army [t(97) = 2.45, p = 0.016]. The statistical analysis also found non-significant differences in digit ratios between members of the Marine Corps and members of the Air Force
Our study hypothesized that individuals with lower digit ratios would tend to choose branches of military service involving training with higher levels of risk than would those with higher digit ratios. We recruited 128 veterans from classes in two Korean universities to explore this issue to provide a sample that was appropriate for statistical analysis. The sample consisted of 128 male participants aged 22–26 years (M = 24.32, SD = 0.71); 58 participants were from the Army, 29 from the Navy and Air Force, and 41 from the Marine Corps. We gathered samples from two universities in Korea because a limited number of student veterans, especially from the Marine Corps, Navy, and Air Force, were available in one university. Because the sample was ethnically homogenous, consisting entirely of Koreans, ethnic differences in 2D:4D (Manning, 2002) were controlled in this study. 2.2. Procedure and measures The current study hypothesized that individuals with higher levels of testosterone would choose military branches involving
Fig. 1. Differences in average digit ratios among members of the marine corps, air force, navy, and army (n = 128).
168
H. Huh / Personality and Individual Differences 53 (2012) 166–168
include different levels of risk and intensity in their training regimens. Acknowledgement This study was supported by Sejong University. I greatly appreciate Yeongtaek Jeong, Seunghoon Kim, Yujin Lee, Minhui Park, Eunbin Jin, and Serim Jeon who helped to develop ideas and gather data. References
Fig. 2. Differences in average digit ratios between marines and non-marine members of the military (n = 128).
[t(58) = 0.805, p = 0.424] and between members of the Marine Corps and members of the Navy [t(50) = 1.107, p = 0.273]. 4. Discussion Our study found evidence for significant group differences in digit ratios between Marines and non-Marine service members, as well as between Marines and members of the Army. As expected, young males who volunteered for the Marine Corps were characterized by significantly lower digit ratios (higher testosterone) than were those who volunteered for the other branches of the military. More specifically, the average digit ratio was 0.94 (SD = 0.05) for the Marine Corps, 0.95 (SD = 0.07) for the Air Force, 0.96 (SD = 0.06) for the Navy, and 0.97 (SD = 0.06) for the Army. Thus, digit ratios are likely to influence even the type of military service chosen by individuals. As our findings demonstrate, physical traits such as digit ratios allow us to predict who has the propensity to take risks in the military. However, this study has several potential limitations. One concern is selection bias. That is, veterans who are college students may not accurately represent the populations of the respective military services. We sampled veterans as participants because security issues prevented the military authorities from helping with our research. Thus, we recommend that future research sample recruits at boot camp instead of focusing on veterans. Additionally, our study focused mainly on the role of physical traits, not cognitive abilities, in decisions about volunteering for different branches of military service. Thus, future research should extend the analysis to the combination of prenatal testosterone and cognitive abilities (Wong, Ormiston, & Haselhuhn, 2011; Hopp, de Moraes, & Jorge, 2012; Branas-Garza & Rustichini, 2011). It is commonly believed that testosterone promotes statusseeking and social dominance (Eisenegger, Haushofer, & Fehr, 2011) and that it also has fear-reducing properties in humans (Mazur, 1985). Individuals who have experienced exposure to high levels of prenatal testosterone may take higher risks to achieve status and social dominance by first reducing or buffering the fear and stress responses that can potentially result from military service. Thus, our findings suggest that individuals with lower digit ratios may demonstrate a more general repertoire of dominance behaviors in the service of non-aggressively achieving or maintaining a high social status, even during military service. We conclude that 2D:4D is linked to choices among Korean military branches, which
Beltz, A. M., Swanson, J. L., & Berenbaum, S. A. (2011). Gendered occupational interests: Prenatal androgen effects on psychological orientation to Things versus People. Hormones and Behavior, 60, 313–317. Branas-Garza, P., & Rustichini, A. (2011). Organising effects of testosterone and economic behaviour: Not just risk taking. PLoS One, 6–12, e29842. Breedlove, S. M. (2010). Minireview: Organizational hypothesis: Instances of the fingerpost. Endocrinology, 151(9), 4116–4122. Brosnan, M. J. (2006). Digit ratio and faculty membership: Implications for the relationship between prenatal testosterone and academia. British Journal of Psychology, 97, 455–466. Dabbs, J. M., Jr., Alford, E. C., & Fielden, J. A. (1998). Trial lawyers and testosterone: Blue-collar talent in a white-collar world. Journal of Applied Social Psychology, 28(1), 84–94. Dabbs, J. M., Jr., de La Rue, D., & Williams, P. M. (1990). Testosterone and occupational interests: Actors, ministers, and other men. Journal of Personality and Social Psychology, 59(6), 1261–1265. Eisenegger, C., Haushofer, J., & Fehr, E. (2011). The role of testosterone in social interaction. Trends in Cognitive Sciences, 15(6), 264–271. Fink, B., Manning, J. T., & Neave, N. (2004). Second to fourth digit ratio and the ‘big five’ personality factors. Personality and Individual Differences, 37(3), 495–503. Hell, B., & PaBler, K. (2011). Are occupational interests hormonally influenced? The 2D:4D – interest nexus. Personality and Individual Differences, 51(4), 376–380. Hopp, R. N., de Moraes, J. P., & Jorge, J. (2012). Digit ratio and academic performance in dentistry students. Personality and Individual Differences, 52(5), 643–646. Lindova, J., Hruskova, M., Pivonkova, V., Kubena, A., & Flegr, J. (2008). Digit ratio (2D:4D) and Cattell’s personality traits. European Journal of Personality, 22, 347–356. Manning, J. T. (2002). Digit ratio: A pointer to fertility, behavior and health. London: Rutgers University Press. Manning, J. T. (2008). The finger book. London: Faber & Faber. Manning, J. T. (2011). Resolving the role of prenatal sex steroids in the development of digit ratio. PNAS, 108(39), 16143–16144. Manning, J. T., Reimers, S., Baron-Cohen, S., Wheelwright, S., & Fink, B. (2010). Sexually dimorphic traits (digit ratio, height, systemizing-empathizing scores) and gender segregation between occupations. Evidence from the BBC internet study. Personality and Individual Differences, 49(5), 511–515. Manning, J. T., Scutt, D., Wilson, J., & Lewis-Jones, D. I. (1998). The ratio of 2nd to 4th digit length: A predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen. Human Reproduction, 13, 3000–3004. Mazur, A. (1985). A biosocial model of status in face-to-face primate groups. Social Forces, 64, 377–402. Ministry of National Defense. (2011). Defense white paper. Republic of Korea: Ministry of National Defense. Stenstrom, E., Saad, G., Nepomuceno, M. V., & Mendenhall, Z. (2011). Testosterone and domain-specific risk: Digit ratios (2D:4D and rel2) as predictors of recreational, financial, and social risk-taking behaviors. Personality and Individual Differences, 51(4), 412–416. Talarovicova, A., Krskova, L., & Blazekova, J. (2009). Testosterone enhancement during pregnancy influences the 2D:4D ratio and open field motor activity of rat siblings in adulthood. Hormones and Behavior, 55(1), 235–239. Voracek, M., Pum, U., & Dressler, S. G. (2010). Investigating digit ratio (2D:4D) in a highly male-oriented occupation: The case of firefighters. Scandinavian Journal of Psychology, 51, 146–156. Weis, S. E., Firker, A., & Hennig, J. (2007). Associations between the second to fourth digit ratio and career interests. Personality and Individual Differences, 43(3), 485–493. White, R. E., Thornhill, S., & Hampson, E. (2007). A biosocial model of entrepreneurship: The combined effects of nurture and nature. Journal of Organizational Behavior, 28, 451–466. Wong, E. M., Ormiston, M. E., & Haselhuhn, M. P. (2011). A face only an investor could love: CEO’s facial structure predicts their firms’ financial performance. Psychological Science, 22(12), 1478–1483. Zhengui, Z., & Martin, J. C. (2011). Developmental basis of sexual dimorphic digit ratios. PNAS, 108(39), 16289–16294.
Contents lists available at SciVerse ScienceDirect
Personality and Individual Differences journal homepage: www.elsevier.com/locate/paid
Born to be a Marine: Digit ratios and military service HaengRyang Huh ⇑ Department of Communication Arts, Sejong University, Chiphyun Bldg 513, 98 Kunja-Dong, Kwangjin-Ku, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 January 2012 Received in revised form 8 February 2012 Accepted 28 February 2012 Available online 25 March 2012 Keywords: Digit ratio Risk-taking Testosterone Occupations Military service Marines Army
a b s t r a c t Digit ratio (2D:4D) is a negative correlate of prenatal testosterone. The current study tested the hypothesis that 2D:4D would be associated with the type of military service chosen by young Korean males. Our sample consisted of 128 young males aged 22–26 years (M = 24.32, SD = 0.89). We measured the 2D:4D on the participants’ right hands and explored its relationship to their selection of one of four branches of military service. Statistical analysis found no significant differences among the four branches of military service but supported our hypothesis of significant group differences in 2D:4D between Marines and all non-Marines. As expected, members of the Marine Corps demonstrated the lowest digit ratios, whereas those in the Army showed the highest. The average 2D:4D was 0.94 (SD = 0.05) for the Marine Corps, 0.95 (SD = 0.07) for the Air Force, 0.96 (SD = 0.06) for the Navy, and 0.97 (SD = 0.06) for the Army. Our research found evidence of small but significant group differences in 2D:4D among members of different branches of military service. We conclude that low 2D:4D is related to the risk and severity associated with the type of military training selected by recruits. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Established studies provide compelling evidence of the close association of occupational choice and achievement with both testosterone level and digit ratio. One previous study demonstrated occupational differences in testosterone concentrations among actors, ministers, firemen, sales people, physicians, football players, and professors (Dabbs, de La Rue, & Williams, 1990). Male and female trial lawyers had testosterone levels that were higher than those of nontrial lawyers of the same sex (Dabbs, Alford, & Fielden, 1998). Individuals with higher testosterone levels and family business backgrounds were more likely to create new business ventures (White, Thornhill, & Hampson, 2007). The literature also provides strong support for hormonal androgen influences on interest in occupations characterized by working with things versus with people (Beltz, Swanson, & Berenbaum, 2011). Likewise, previous studies have demonstrated a significant relationship between occupational choice and digit ratio. Low 2D:4D (the relative lengths of the second and fourth fingers), indicating high levels of prenatal testosterone, was related to a ‘‘masculinized’’ score for career interests in groups of both males and females according to different scales (Weis, Firker, & Hennig, 2007). Significant differences in digit ratios among faculty in science and social science departments have also been demonstrated (Brosnan, 2006). Additionally, significant correlations of finger length (positive) and digit ratio (negative) with realistic interests ⇑ Corresponding author. Tel.: +82 2 3408 3707; fax: +82 2 3408 4307. E-mail addresses: [email protected], [email protected] 0191-8869/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.paid.2012.02.021
and significant correlations between finger length (negative) and social interests, a marker of the people–things dimension, have also been reported (Hell & PaBler, 2011). Manning, Reimers, Baron-Cohen, Wheelwright, and Fink (2010) reported that low 2D:4D was found in individuals in male-dominated occupations; this finding was particularly robust among females. These studies addressed career interests but not aspects of risk taking. Manning, Scutt, Wilson, and Lewis-Jones (1998) suggested that 2D:4D ratio was negatively correlated with prenatal testosterone. Evidence for this hypothesis was subsequently reviewed by Manning (2002, 2008) and Breedlove (2010). Furthermore, recent experimental work on the fetuses of mammals has essentially confirmed that the 2D:4D ratio is negatively associated with prenatal testosterone and positively associated with prenatal estrogen (Manning, 2011; Talarovicova, Krskova, & Blazekova, 2009; Zhengui & Martin, 2011). If digit ratios were taken as an indicator of prenatal exposure to testosterone, the relationship between individual thoughts and behaviors and digit ratios should follow the same pattern as the relationship between these phenomena and testosterone. Indeed, digit ratios offer a valid test of the organizational hypothesis that androgens act early in life to masculinize various human behaviors (Breedlove, 2010). Previous research investigating the relationships between digit ratios (2D:4D) and the propensity of individuals to engage in risky and aggressive behavior has revealed associations between digit ratios and personality traits (Fink, Manning, & Neave, 2004; Lindova, Hruskova, Pivonkova, Kubena, & Flegr, 2008). Digit ratios have also been associated with recreational, financial, and social risk-taking behaviors (Stenstrom,
H. Huh / Personality and Individual Differences 53 (2012) 166–168
Saad, Nepomuceno, & Mendenhall, 2011). Thus, digit ratios have been linked to individuals’ risk taking. Testosterone and risk taking are closely related, and evidence has suggested an association between 2D:4D and risk taking. Korea requires a specific amount of military service from every male citizen. Some young males in Korea volunteer to be members of the Marine Corps, Navy, or Air Force in advance, whereas most males wait to be conscripted by the Army. Military branches vary according to length of service and severity of training. The lengths of service are 21 months for the Army and Marine Corps, 23 months for the Navy, and 24 months for the Air Force (Ministry of National Defense, 2011). The four options for military service involve different degrees of severity in the requisite training. It is commonly believed that the Navy and Air Force require the mildest training, whereas the Marine Corps requires the harshest. Different types of military services also require individuals to engage in different levels of risk depending on length of service and severity of training. Individuals usually differ with regard to level of risk-taking behavior, including military service. Most studies have not focused on the relationship between risk taking and digit ratios. Indeed, one previous study provided no evidence for this perspective, showing instead that 2D:4D in firefighters (N = 134) was not lower than that of controls drawn from the local male population (Voracek, Pum, & Dressler, 2010). However, this study also reported several other interesting findings, including an association between low 2D:4D and low rank and the absence of evidence for a link between low 2D:4D and superior performance in sports. These conclusions are so unexpected that they may indicate that the reported findings are unreliable. Thus, this study explored the relationship between digit ratio and military service. Individuals differ with respect to the propensity to take risks, which suggests that digit ratios may be associated with the probability of volunteering for different branches of military service. Individuals with lower digit ratios may tend to prefer higher-risk military branches than would those with higher digit ratios. We hypothesized that individuals with lower digit ratios would tend to prefer branches with more severe training than would those with higher ratios. The current study was designed to explore the relationship between digit ratios and choices among different branches of military service. 2. Method 2.1. Participants
167
greater risk than would individuals with lower levels of testosterone. First, participants were asked to complete questionnaires that solicited information on demographic variables such as age and type of military service. Before distributing the survey, we used a focus group to explore the degree of risk taking in four types of military service. The focus group was composed of 7 student veterans. This focus group of seven veterans rated the Marine Corps highest on the spectrum of risk taking; the Navy and Air Force followed, and the Army was rated lowest. Second, we photographed participants’ hands to determine digit ratios. We included the traditional 2D:4D measure as a proxy for prenatal androgen exposure. The current study focused on digit ratios in the right hand because this hand tends to be more sex dimorphic (Manning, 2002). Finger lengths were measured from the ventral proximal crease to the fingertip by using Adobe Photoshop ruler. To check for reliability, a second independent rater also measured finger lengths. The inter-rater correlation coefficient for the two measurements of 2D:4D was 0.86 (p < 0.001), which indicated a high level of reliability.
3. Results The average digit ratio for all participants was 0.96 (SD = 0.06). First, we used a one-way analysis of variance to explore group differences in digit ratios among members of the Marine Corps, Navy, Air Force, and Army. As Fig. 1 demonstrates, the average digit ratio was 0.94 (SD = 0.05) for the Marine Corps, 0.95 (SD = 0.07) for the Air Force, 0.96 (SD = 0.06) for the Navy, and 0.97 (SD = 0.06) for the Army. A one-way analysis of variance demonstrated non-significant group differences in digit ratios among members of the four branches of military service [F(2, 125) = 1.822, p = 0.147]. Second, the statistical analysis demonstrated significant group differences in digit ratios between Marines and non-Marines. As illustrated by Fig. 2, the average digit ratio for Marines was 0.94 (SD = 0.05), whereas that for non-Marines was 0.96 (SD = 0.06). Statistical analysis demonstrated significant group differences in digit ratios between Marines and non-Marines [t(127) = 2.33, p = 0.02]. As expected, Marines had lower digit ratios than did non-Marines. We then explored whether significant difference between the Marine Corps and each of the other military services could be observed. First, statistical analysis demonstrated a significant difference in digit ratios between members of the Marine Corps and members of the Army [t(97) = 2.45, p = 0.016]. The statistical analysis also found non-significant differences in digit ratios between members of the Marine Corps and members of the Air Force
Our study hypothesized that individuals with lower digit ratios would tend to choose branches of military service involving training with higher levels of risk than would those with higher digit ratios. We recruited 128 veterans from classes in two Korean universities to explore this issue to provide a sample that was appropriate for statistical analysis. The sample consisted of 128 male participants aged 22–26 years (M = 24.32, SD = 0.71); 58 participants were from the Army, 29 from the Navy and Air Force, and 41 from the Marine Corps. We gathered samples from two universities in Korea because a limited number of student veterans, especially from the Marine Corps, Navy, and Air Force, were available in one university. Because the sample was ethnically homogenous, consisting entirely of Koreans, ethnic differences in 2D:4D (Manning, 2002) were controlled in this study. 2.2. Procedure and measures The current study hypothesized that individuals with higher levels of testosterone would choose military branches involving
Fig. 1. Differences in average digit ratios among members of the marine corps, air force, navy, and army (n = 128).
168
H. Huh / Personality and Individual Differences 53 (2012) 166–168
include different levels of risk and intensity in their training regimens. Acknowledgement This study was supported by Sejong University. I greatly appreciate Yeongtaek Jeong, Seunghoon Kim, Yujin Lee, Minhui Park, Eunbin Jin, and Serim Jeon who helped to develop ideas and gather data. References
Fig. 2. Differences in average digit ratios between marines and non-marine members of the military (n = 128).
[t(58) = 0.805, p = 0.424] and between members of the Marine Corps and members of the Navy [t(50) = 1.107, p = 0.273]. 4. Discussion Our study found evidence for significant group differences in digit ratios between Marines and non-Marine service members, as well as between Marines and members of the Army. As expected, young males who volunteered for the Marine Corps were characterized by significantly lower digit ratios (higher testosterone) than were those who volunteered for the other branches of the military. More specifically, the average digit ratio was 0.94 (SD = 0.05) for the Marine Corps, 0.95 (SD = 0.07) for the Air Force, 0.96 (SD = 0.06) for the Navy, and 0.97 (SD = 0.06) for the Army. Thus, digit ratios are likely to influence even the type of military service chosen by individuals. As our findings demonstrate, physical traits such as digit ratios allow us to predict who has the propensity to take risks in the military. However, this study has several potential limitations. One concern is selection bias. That is, veterans who are college students may not accurately represent the populations of the respective military services. We sampled veterans as participants because security issues prevented the military authorities from helping with our research. Thus, we recommend that future research sample recruits at boot camp instead of focusing on veterans. Additionally, our study focused mainly on the role of physical traits, not cognitive abilities, in decisions about volunteering for different branches of military service. Thus, future research should extend the analysis to the combination of prenatal testosterone and cognitive abilities (Wong, Ormiston, & Haselhuhn, 2011; Hopp, de Moraes, & Jorge, 2012; Branas-Garza & Rustichini, 2011). It is commonly believed that testosterone promotes statusseeking and social dominance (Eisenegger, Haushofer, & Fehr, 2011) and that it also has fear-reducing properties in humans (Mazur, 1985). Individuals who have experienced exposure to high levels of prenatal testosterone may take higher risks to achieve status and social dominance by first reducing or buffering the fear and stress responses that can potentially result from military service. Thus, our findings suggest that individuals with lower digit ratios may demonstrate a more general repertoire of dominance behaviors in the service of non-aggressively achieving or maintaining a high social status, even during military service. We conclude that 2D:4D is linked to choices among Korean military branches, which
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