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Neuroticism and conscientiousness are associated with cortisol diurnal profiles in adults—Role of positive and negative affect
Psychoneuroendocrinology (2010) 35, 1573—1577
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p s y n e u e n
SHORT COMMUNICATION
Neuroticism and conscientiousness are associated with cortisol diurnal profiles in adults–—Role of positive and negative affect Urs M. Nater a,*, Christiane Hoppmann b, Petra L. Klumb c a
University of Zurich, Dept. of Psychology, Switzerland University of British Columbia, Dept. of Psychology, Vancouver, BC, Canada c University of Fribourg, Dept. of Psychology, Switzerland b
Received 3 August 2009; received in revised form 22 February 2010; accepted 23 February 2010
KEYWORDS Neuroticism; Conscientiousness; Cortisol; Affect
Summary A substantial body of research on the pathophysiology of negative health outcomes has focused on dysregulation of the hypothalamic—pituitary—adrenal (HPA) axis. Maladaptive and adaptive personality features have been discussed to be associated with health outcomes. In the current study, we investigated the association of neuroticism (N) and conscientiousness (C) with diurnal cortisol levels in 102 working parents (M age = 37 years; 50% female). Further, we examined the impact of daily positive and negative affect on this association. During a 6-day time-sampling phase, cortisol was measured at awakening and after that within intervals of 3 h. We found a positive association of N with cortisol levels throughout the measurement period, but no association of C with daily cortisol. When accounting for daily positive and negative affect, individuals with high scores on C displayed reductions in daily cortisol concentrations that were driven by positive affect compared to individuals with low C scores. No such association emerged for N. Our findings might further elucidate the role of personality in HPA axis regulation and improve our understanding of the association of endocrine states and health outcomes. # 2010 Elsevier Ltd. All rights reserved.
1. Introduction A substantial body of research on the pathophysiology of negative health outcomes has focused on dysregulation of the hypothalamic—pituitary—adrenal (HPA) axis. This system has
* Corresponding author at: University of Zurich, Clinical Psychology & Psychotherapy, Binzmuehlestr. 14/Box 26, 8050 Zurich, Switzerland. Tel.: +41 44 635 7382; fax: +41 44 635 7359. E-mail address: [email protected] (U.M. Nater).
been of primary interest, because HPA axis hormones, such as cortisol, may contribute to the peripheral and central causes of a number of symptoms, such as pain and fatigue (Clauw and Chrousos, 1997). Cortisol affects glucose production, fat metabolism, inflammatory responses, cardiovascular responsiveness, and central nervous system and immune functioning (Charmandari et al., 2003; de Kloet, 2003). Maladaptive and adaptive personality features have been discussed to be associated with health outcomes. Using a dimensional approach, personality can be best described by the five-factor model, which provides a structure of normal
0306-4530/$ — see front matter # 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.psyneuen.2010.02.017
1574 personality traits. Particularly, neuroticism (N) or the tendency to experience negative affect, affective instability, and increased stress levels, has been associated with negative health outcomes or to contribute to the maintenance of illness (Costa and McCrae, 1987; Appel et al., 2007). On the other hand, conscientiousness (C) or the tendency to be organized, thorough, and reliable, has been shown to be linked to positive health outcomes (Kern and Friedman, 2008). A small body of research has related measurement of personality to basal HPA axis activity. These studies focused on cortisol measurement as an index of HPA axis regulation. Most studies report on the association of N and cortisol. It might be assumed that N, reflecting a tendency to experience increased stress levels, is associated with increased cortisol levels throughout the day. However, previous studies remained inconclusive, as some of them found increased (Portella et al., 2005; Wetherell et al., 2006) or decreased cortisol concentrations (Saey et al., 2005), or no differences at all (Schommer et al., 1999) in subjects scoring high on N compared to individuals with low scores in N. Regarding C, it could be hypothesized that subjects scoring high in C are more organized when it comes to daily tasks and thus display lower cortisol levels throughout the day. However, we are not aware of any studies relating C to basal cortisol concentrations. A limitation of previous studies is measurement of cortisol at a single point in time or during a single day. Considering the variability of cortisol concentrations both within and between days, measurement of cortisol should go beyond being just a snapshot in time (Hellhammer et al., 2007). This is particularly important considering the fact that personality traits such as N or C represent consistent patterns of intraindividual variability (Fleeson, 2004). A recent study has accounted for this fact and, using a momentary assessment measurement approach, found flatter cortisol profiles in male adolescents scoring high in N (Hauner et al., 2008). In the current study, we aimed to extend previous research by linking measurement of N and C with cortisol concentrations measured over several days in adults. We hypothesized that N was positively and C was negatively associated with cortisol concentrations both between and within days (main effect). Considering that basal cortisol concentrations may be influenced by mood changes (Adam et al., 2006), and that N and C may influence mood changes, our second aim was to examine the impact of N and C on daily fluctuations in mood and their association with cortisol concentrations (interaction effect).
2. Materials and methods 2.1. Study design and data collection Participants first completed an internet questionnaire including sociodemographic and personality characteristics. They then entered a 6-day time-sampling phase during which they completed one daily self-initiated questionnaire when waking up and 5 daily prompted questionnaires approximately every 3 h. We used Psion 3a pocket computers as data collection devices. Parallel to questionnaire completion, participants provided saliva samples.
U.M. Nater et al.
2.2. Study group The sample consisted of 102 German working parents from the Berlin Metropolitan area (M age = 37 years; 50% female; M number of children = 1.7; 90% university degree). Eligibility criteria required that both partners of a couple participated and that neither of them had illnesses or took medication that influences the HPA axis. Another criterion was a university degree or comparable level of education. We excluded pregnant and breastfeeding women. For more details on sample and design see Hoppmann and Klumb (2006).
2.3. Assessments Personality: We used six items from the NEO-FFI to assess neuroticism and conscientiousness at baseline (Costa and McCrae, 1985). Participants responded on a 5-point scale (1 (not at all) to 5 (very much)). Mean neuroticism was 1.99 (SD = .60). Mean conscientiousness was 3.82 (SD = .51). Daily affect: At each measurement occasion, participants rated their current positive (good, alert, relaxed) and negative affect (bad, tired, fidgety) on a 5-point rating scale (1 (not at all) to 5 (very much)) (Steyer et al., 1997). Mean daily positive affect was 3.47 (SD = .40). Mean daily negative affect was 1.89 (SD = .49). Cortisol: Saliva samples were collected using Salivettes (Sarstedt Rommelsdorf, Germany). All participants were asked to provide their first sample after awakening while still lying in bed (mean collection time 07:06 am). Further samples were taken at 09:00 am, 12:00 pm, 03:00 pm, 06:00 pm, and 09:00 pm. Participants were given the option to deviate from the set time points to reduce scheduling conflicts (e.g. important meetings). Moving the time point by 15 min typically resolved scheduling conflicts. Cortisol was analyzed using a commercial chemiluminescence immunoassay (IBL Hamburg, Germany). For the purpose of this study we concentrated on estimates of the total daily cortisol secretion based on the ‘‘area under the curve’’ (AUC) (Pruessner et al., 2003). The mean daily AUC was 6643.68 nmol/l (SD = 1618.42 nmol/l). Control variables: We also controlled for the influence of smoking habits, hormonal contraceptive use, exercise, body mass index, sleep duration, wake-up time, and depressive symptoms.
2.4. Statistical analysis Multilevel modeling was used to account for the hierarchical nature of this data set (HLM, Raudenbush et al., 2000). Specifically, we conducted models with three levels of nesting. The first level concerned daily assessments, the second level participants, and the third level couples.
3. Results 3.1. Personality and daily cortisol secretion As a first step we examined the association between individual differences in N and C and daily AUCs, including several important control variables (see Table 1, Model 1). Findings
Personality and daily cortisol
1575
Table 1 Hierarchical linear models predicting daily cortisol secretion from neuroticism and conscientiousness, and daily affect quality using full maximum likelihood estimation in HLM (N = 104). Effect (se)
Model 1: main effects
Fixed effects estimates Intercept CES-D Hormonal contraceptives Smoking Body mass index Physical activity Sleep duration Wake-up time Sex Neuroticism Conscientiousness Positive affect Negative affect Positive affect neuroticism Positive affect conscientiousness Random effects estimates Residual variance Intercept lv 1 variance Intercept lv 2 variance Deviance reduction (based on FML; (df) * **
6683.06** (269.33) 520.49 (443.47) 128.38 (431.52) 293.28 (235.01) 122.05* (55.51) 169.55* (81.91) 1.07 (3.49) 7.13 (4.82) 148.95 (352.32) 760.00* (313.31) 58.17 (263.46)
4,819,636.24 662,801.67 715,532.77 20.87* (10)
Model 2: interaction effects 6690.23** (271.96) 494.10 (441.10) 208.19 (430.98) 294.05 (235.00) 123.02* (56.76) 164.18* (81.04) .62 (3.48) 6.71 (4.80) 182.82 (364.40) 755.59* (320.52) 69.39 (262.90) 2184.94** (452.43) 313.68 (445.20) 611.34 (578.34) 1402.68* (638.22) 3,999,483.25 825,703.77 681,411.20 129.09** (16)
p < .05. p < .01.
indicate that N was positively associated with higher mean daily AUCs whereas C was unrelated to mean daily AUCs. This means that participants with high N scores displayed elevated daily cortisol levels as compared to individuals with low N scores. The model explained 6.5% of the variance in daily cortisol levels.
mean daily positive affect and lower mean daily negative affect than individuals with low C. Hence, the two personality characteristics were meaningfully related to daily affect and showed distinct associations with daily affect. Model 1 explained 10.9% of the variance in daily positive affect. Model 2 explained 18.04% of the variance in daily negative affect.
3.2. Personality and daily affect 3.3. Personality, daily affect, and cortisol levels We next estimated the relationship between N and C and daily affect (Table 2, Models 1 and 2). Results show that individuals scoring high on N reported lower mean daily positive affect and higher mean daily negative affect than individuals low in N. Individuals with high C, reported higher
A final aim of this study was to investigate more complex associations between personality, daily affect, and cortisol levels. Specifically, we wanted to know whether personality interacted with daily affect to predict daily AUCs. Results are
Table 2 Hierarchical linear models predicting daily affect quality from neuroticism and conscientiousness using full maximum likelihood estimation in HLM (N = 104). Effect (se) Fixed effects estimates Intercept Sex Neuroticism Conscientiousness Random effects estimates Residual variance Intercept lv 1 variance Intercept lv 2 variance Deviance reduction (based on FML; (df) * **
p < .05. p < .01.
Model 1: positive affect 3.48** (.05) .03 (.06) .22** (.05) .15* (.07) .15 .06 .04 23.84** (3)
Model 2: negative affect 1.88** (.06) .05 (.06) .22** (.08) .32** (.09) .11 .09 .07 30.25** (3)
1576
Figure 1 Interaction of conscientiousness and positive affect on cortisol area under the curve (AUC).
shown in Table 1 (Model 2). Our results indicate again that N was positively associated with mean daily AUCs. In addition, we also found within-person associations between daily positive affect and daily AUCs. On days when participants reported higher positive affect they where more likely to display lower AUCs as compared to days characterized by lower positive affect. No such association emerged for daily negative affect. Importantly, we further detected a significant interaction between C and daily positive affect on daily AUCs. Hence, the effect of positive affect on daily AUCs was stronger for individuals high in C. This interaction is illustrated Fig. 1 by contrasting positive affect-AUC associations in individuals high in C (1 SD above the mean) with individuals low in C (1 SD below the mean). No interaction was observed regarding C and negative affect or N and daily affect quality. Model 2 explained 13.65% of the variance in daily cortisol secretion.
4. Discussion We found that higher scores in N were associated with higher levels of cortisol throughout the measurement period, whereas no main effect of C on daily cortisol was found. When accounting for daily fluctuations in positive and negative affect, we found that individuals with high scores on C displayed reductions in daily cortisol concentrations that were driven by positive affect compared to individuals with low C scores. No such association emerged for N. We were thus able to show that N exerts a direct effect on daily cortisol levels, whereas C moderates the association between daily positive affect and cortisol. Our results are in accordance with other studies reporting on a positive relationship between N and basal cortisol (Portella et al., 2005; Wetherell et al., 2006). It needs to be critically noted, though, that these studies employed only one measurement time point and/or only one collection day. We present here the first study in which basal cortisol has been sampled over an extended period of time. Our results in adults complement previous findings of a study in adolescents using a similar assessment method but different cortisol indices (Hauner et al., 2008). Specifically, these authors found a negative association between N and daily cortisol slopes (in male participants only). The positive association in our study
U.M. Nater et al. might explain to some extent how high N might be associated with negative health outcomes (Lahey, 2009): N is suggested to exert its effect via increased basal HPA axis activity (as indicated by high cortisol levels over a certain period of time), which might lead to allostatic load and, ultimately, to dysfunction in metabolic, immune, and cardiovascular systems (Seeman and Gruenewald, 2006; Chrousos, 2009). To the best of our knowledge, no study so far has examined the relationship between C and daily cortisol. Importantly, the association between C and cortisol was moderated by positive affect in our study. This finding bears important consequences for a better understanding of how high scores in C translate into positive health outcomes, as a variety of studies have shown (Kern and Friedman, 2008). It might be proposed that C exerts its positive influence on health via affect-related regulation of HPA axis activity. In particular, it could be hypothesized that individuals with high scores in C tend to be more organized regarding their daily life matters and are thus less likely to experience distress by challenging everyday life tasks. Our findings need to be considered in the light of a few limitations. First, we have examined associations of personality and basal cortisol. A variety of studies have used dynamic measurement of cortisol by using pharmacological (McCleery et al., 2000; Zobel et al., 2004) or psychological (Phillips et al., 2005; Oswald et al., 2006) challenge tests. Considering that the effects of N in particular might be most pronounced in a situation of stress reactivity, future studies should emphasize measurement of dynamic tests and implement assessment of affect. Second, the effect of N and C on health outcomes might not only be moderated by affect, but also by a range of health behaviors (Hagger-Johnson and Whiteman, 2007). However, we controlled for a spectrum of health behaviors, such as exercise, sleep, and smoking in the current study, and our findings were not affected by these behaviors. Third, our study is based on 1-week time-sampling information that captures multiple interrelated processes. As a result, causal inferences are not allowed. Experimental research would complement our approach by testing the respective mechanisms under controlled laboratory conditions. Fourth, we did not control for effects of food intake on cortisol levels. Future studies will need to incorporate measurement of food intake in order to control for this potential confound. Finally, effect sizes are relatively small. Although this is true for most time-sampling research, it is still important to point out that the data capture a multitude of complex daily life processes that are difficult to control for (Hoppmann and Riediger, 2009). In conclusion, the findings of this study have the potential to further elucidate the role of personality in HPA axis regulation and, thus, improve our understanding of the association of endocrine states and health outcomes.
Role of the funding sources The funding agency had no role in data collection, data analysis, or writing of the manuscript.
Conflict of Interest The authors declare no conflict of interest.
Personality and daily cortisol Disclosure Information: The authors declare no financial interest related to the study.
Acknowledgments The research reported here was funded by a Volkswagen Foundation grant to Petra Klumb.
References Adam, E.K., Hawkley, L.C., Kudielka, B.M., Cacioppo, J.T., 2006. Day-to-day dynamics of experience–—cortisol associations in a population-based sample of older adults. Proc. Natl. Acad. Sci. U.S.A. 103, 17058—17063. Appel, S., Chapman, J., Shoenfeld, Y., 2007. Infection and vaccination in chronic fatigue syndrome: myth or reality? Autoimmunity 40, 48—53. Charmandari, E., Kino, T., Souvatzoglou, E., Chrousos, G.P., 2003. Pediatric stress: hormonal mediators and human development. Horm. Res. 59, 161—179. Chrousos, G.P., 2009. Stress and disorders of the stress system. Nat. Rev. Endocrinol.. Clauw, D.J., Chrousos, G.P., 1997. Chronic pain and fatigue syndromes: overlapping clinical and neuroendocrine features and potential pathogenic mechanisms. Neuroimmunomodulation 4, 134—153. Costa Jr., P.T., McCrae, R.R., 1987. Neuroticism, somatic complaints, and disease: is the bark worse than the bite? J. Pers. 55, 299—316. Costa Jr., P.T., McCrae, R.R., 1985. The NEO Personality Inventory manual. Psychological Assessment Resources, Odessa FL. de Kloet, E.R, 2003. Hormones, brain and stress. Endocr. Regul. 37, 51—68. Fleeson, W., 2004. Moving personality beyond the person—situation debate: the challenge and the opportunity of within-person variability. Curr. Dir. Psychol. Sci. 13, 83—87. Hagger-Johnson, G.E., Whiteman, M.C., 2007. Conscientiousness facets and health behaviors: a latent variable modeling approach. Pers. Indiv. Differ. 43, 1235—1245. Hauner, K.K., Adam, E.K., Mineka, S., Doane, L.D., DeSantis, A.S., Zinbarg, R., Craske, M., Griffith, J.W., 2008. Neuroticism and introversion are associated with salivary cortisol patterns in adolescents. Psychoneuroendocrinology 33, 1344—1356. Hellhammer, J., Fries, E., Schweisthal, O.W., Schlotz, W., Stone, A.A., Hagemann, D., 2007. Several daily measurements are necessary to reliably assess the cortisol rise after awakening: state- and trait components. Psychoneuroendocrinology 32, 80—86. Hoppmann, C.A., Klumb, P.L., 2006. Daily goal pursuits predict cortisol secretion and mood states in employed parents with preschool children. Psychosom. Med. 68, 887—894. Hoppmann, C.A., Riediger, M., 2009. Ambulatory assessment in lifespan psychology: an overview of current status and new trends. Eur. J. Psychol. 14, 98—108.
1577 Kern, M.L., Friedman, H.S., 2008. Do conscientious individuals live longer? A quantitative review. Health Psychol. 27, 505— 512. Lahey, B.B., 2009. Public health significance of neuroticism. Am. Psychol. 64, 241—256. McCleery, J.M., Bhagwagar, Z., Smith, K.A., Goodwin, G.M., Cowen, P.J., 2000. Modelling a loss event: effect of imagined bereavement on the hypothalamic—pituitary—adrenal axis. Psychol. Med. 30, 219—223. Oswald, L.M., Zandi, P., Nestadt, G., Potash, J.B., Kalaydjian, A.E., Wand, G.S., 2006. Relationship between cortisol responses to stress and personality. Neuropsychopharmacology. Phillips, A.C., Carroll, D., Burns, V.E., Drayson, M., 2005. Neuroticism, cortisol reactivity, and antibody response to vaccination. Psychophysiology 42, 232—238. Portella, M.J., Harmer, C.J., Flint, J., Cowen, P., Goodwin, G.M., 2005. Enhanced early morning salivary cortisol in neuroticism. Am. J. Psychiatry 162, 807—809. Pruessner, J.C., Kirschbaum, C., Meinlschmidt, G., Hellhammer, D.H., 2003. Two formulas for computation of the area under the curve represent measures of total hormone concentration versus time-dependent change. Psychoneuroendocrinology 28, 916—931. Raudenbush, S.W., Bryk, A.S., Cheong, Y.F., Congdon, R., 2000. HLM 5. Hierarchal linear and nonlinear modeling. Scientific Software International, Chicago, IL. Saey, D., Michaud, A., Couillard, A., Cote, C.H., Mador, M.J., Leblanc, P., Jobin, J., Maltais, F., 2005. Contractile fatigue, muscle morphometry, and blood lactate in chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 171, 1109— 1115. Schommer, N.C., Kudielka, B.M., Hellhammer, D.H., Kirschbaum, C., 1999. No evidence for a close relationship between personality traits and circadian cortisol rhythm or a single cortisol stress response. Psychol. Rep. 84, 840—842. Seeman, T.E., Gruenewald, T.L., 2006. Allostasis and allostatic load over the life course. In: van Lange, P.A.M. (Ed.), Medical and Psychiatric Comorbidity over the Course of Life. American Psychiatric Publishers, Washington, DC, pp. 179— 196. Steyer, R., Schwenkmezger, P., Notz, P., Eid, M., 1997. Der Mehrdimensionale Befindlichkeitsfragebogen (MDBF). Go ¨ttingen, Hogrefe. Wetherell, M.A., Crown, A.L., Lightman, S.L., Miles, J.N., Kaye, J., Vedhara, K., 2006. The four-dimensional stress test: psychological, sympathetic-adrenal—medullary, parasympathetic and hypothalamic—pituitary—adrenal responses following inhalation of 35% CO2. Psychoneuroendocrinology 31, 736—747. Zobel, A., Barkow, K., Schulze-Rauschenbach, S., Von Widdern, O., Metten, M., Pfeiffer, U., Schnell, S., Wagner, M., Maier, W., 2004. High neuroticism and depressive temperament are associated with dysfunctional regulation of the hypothalamic—pituitary— adrenocortical system in healthy volunteers. Acta Psychiatr. Scand. 109, 392—399.
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p s y n e u e n
SHORT COMMUNICATION
Neuroticism and conscientiousness are associated with cortisol diurnal profiles in adults–—Role of positive and negative affect Urs M. Nater a,*, Christiane Hoppmann b, Petra L. Klumb c a
University of Zurich, Dept. of Psychology, Switzerland University of British Columbia, Dept. of Psychology, Vancouver, BC, Canada c University of Fribourg, Dept. of Psychology, Switzerland b
Received 3 August 2009; received in revised form 22 February 2010; accepted 23 February 2010
KEYWORDS Neuroticism; Conscientiousness; Cortisol; Affect
Summary A substantial body of research on the pathophysiology of negative health outcomes has focused on dysregulation of the hypothalamic—pituitary—adrenal (HPA) axis. Maladaptive and adaptive personality features have been discussed to be associated with health outcomes. In the current study, we investigated the association of neuroticism (N) and conscientiousness (C) with diurnal cortisol levels in 102 working parents (M age = 37 years; 50% female). Further, we examined the impact of daily positive and negative affect on this association. During a 6-day time-sampling phase, cortisol was measured at awakening and after that within intervals of 3 h. We found a positive association of N with cortisol levels throughout the measurement period, but no association of C with daily cortisol. When accounting for daily positive and negative affect, individuals with high scores on C displayed reductions in daily cortisol concentrations that were driven by positive affect compared to individuals with low C scores. No such association emerged for N. Our findings might further elucidate the role of personality in HPA axis regulation and improve our understanding of the association of endocrine states and health outcomes. # 2010 Elsevier Ltd. All rights reserved.
1. Introduction A substantial body of research on the pathophysiology of negative health outcomes has focused on dysregulation of the hypothalamic—pituitary—adrenal (HPA) axis. This system has
* Corresponding author at: University of Zurich, Clinical Psychology & Psychotherapy, Binzmuehlestr. 14/Box 26, 8050 Zurich, Switzerland. Tel.: +41 44 635 7382; fax: +41 44 635 7359. E-mail address: [email protected] (U.M. Nater).
been of primary interest, because HPA axis hormones, such as cortisol, may contribute to the peripheral and central causes of a number of symptoms, such as pain and fatigue (Clauw and Chrousos, 1997). Cortisol affects glucose production, fat metabolism, inflammatory responses, cardiovascular responsiveness, and central nervous system and immune functioning (Charmandari et al., 2003; de Kloet, 2003). Maladaptive and adaptive personality features have been discussed to be associated with health outcomes. Using a dimensional approach, personality can be best described by the five-factor model, which provides a structure of normal
0306-4530/$ — see front matter # 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.psyneuen.2010.02.017
1574 personality traits. Particularly, neuroticism (N) or the tendency to experience negative affect, affective instability, and increased stress levels, has been associated with negative health outcomes or to contribute to the maintenance of illness (Costa and McCrae, 1987; Appel et al., 2007). On the other hand, conscientiousness (C) or the tendency to be organized, thorough, and reliable, has been shown to be linked to positive health outcomes (Kern and Friedman, 2008). A small body of research has related measurement of personality to basal HPA axis activity. These studies focused on cortisol measurement as an index of HPA axis regulation. Most studies report on the association of N and cortisol. It might be assumed that N, reflecting a tendency to experience increased stress levels, is associated with increased cortisol levels throughout the day. However, previous studies remained inconclusive, as some of them found increased (Portella et al., 2005; Wetherell et al., 2006) or decreased cortisol concentrations (Saey et al., 2005), or no differences at all (Schommer et al., 1999) in subjects scoring high on N compared to individuals with low scores in N. Regarding C, it could be hypothesized that subjects scoring high in C are more organized when it comes to daily tasks and thus display lower cortisol levels throughout the day. However, we are not aware of any studies relating C to basal cortisol concentrations. A limitation of previous studies is measurement of cortisol at a single point in time or during a single day. Considering the variability of cortisol concentrations both within and between days, measurement of cortisol should go beyond being just a snapshot in time (Hellhammer et al., 2007). This is particularly important considering the fact that personality traits such as N or C represent consistent patterns of intraindividual variability (Fleeson, 2004). A recent study has accounted for this fact and, using a momentary assessment measurement approach, found flatter cortisol profiles in male adolescents scoring high in N (Hauner et al., 2008). In the current study, we aimed to extend previous research by linking measurement of N and C with cortisol concentrations measured over several days in adults. We hypothesized that N was positively and C was negatively associated with cortisol concentrations both between and within days (main effect). Considering that basal cortisol concentrations may be influenced by mood changes (Adam et al., 2006), and that N and C may influence mood changes, our second aim was to examine the impact of N and C on daily fluctuations in mood and their association with cortisol concentrations (interaction effect).
2. Materials and methods 2.1. Study design and data collection Participants first completed an internet questionnaire including sociodemographic and personality characteristics. They then entered a 6-day time-sampling phase during which they completed one daily self-initiated questionnaire when waking up and 5 daily prompted questionnaires approximately every 3 h. We used Psion 3a pocket computers as data collection devices. Parallel to questionnaire completion, participants provided saliva samples.
U.M. Nater et al.
2.2. Study group The sample consisted of 102 German working parents from the Berlin Metropolitan area (M age = 37 years; 50% female; M number of children = 1.7; 90% university degree). Eligibility criteria required that both partners of a couple participated and that neither of them had illnesses or took medication that influences the HPA axis. Another criterion was a university degree or comparable level of education. We excluded pregnant and breastfeeding women. For more details on sample and design see Hoppmann and Klumb (2006).
2.3. Assessments Personality: We used six items from the NEO-FFI to assess neuroticism and conscientiousness at baseline (Costa and McCrae, 1985). Participants responded on a 5-point scale (1 (not at all) to 5 (very much)). Mean neuroticism was 1.99 (SD = .60). Mean conscientiousness was 3.82 (SD = .51). Daily affect: At each measurement occasion, participants rated their current positive (good, alert, relaxed) and negative affect (bad, tired, fidgety) on a 5-point rating scale (1 (not at all) to 5 (very much)) (Steyer et al., 1997). Mean daily positive affect was 3.47 (SD = .40). Mean daily negative affect was 1.89 (SD = .49). Cortisol: Saliva samples were collected using Salivettes (Sarstedt Rommelsdorf, Germany). All participants were asked to provide their first sample after awakening while still lying in bed (mean collection time 07:06 am). Further samples were taken at 09:00 am, 12:00 pm, 03:00 pm, 06:00 pm, and 09:00 pm. Participants were given the option to deviate from the set time points to reduce scheduling conflicts (e.g. important meetings). Moving the time point by 15 min typically resolved scheduling conflicts. Cortisol was analyzed using a commercial chemiluminescence immunoassay (IBL Hamburg, Germany). For the purpose of this study we concentrated on estimates of the total daily cortisol secretion based on the ‘‘area under the curve’’ (AUC) (Pruessner et al., 2003). The mean daily AUC was 6643.68 nmol/l (SD = 1618.42 nmol/l). Control variables: We also controlled for the influence of smoking habits, hormonal contraceptive use, exercise, body mass index, sleep duration, wake-up time, and depressive symptoms.
2.4. Statistical analysis Multilevel modeling was used to account for the hierarchical nature of this data set (HLM, Raudenbush et al., 2000). Specifically, we conducted models with three levels of nesting. The first level concerned daily assessments, the second level participants, and the third level couples.
3. Results 3.1. Personality and daily cortisol secretion As a first step we examined the association between individual differences in N and C and daily AUCs, including several important control variables (see Table 1, Model 1). Findings
Personality and daily cortisol
1575
Table 1 Hierarchical linear models predicting daily cortisol secretion from neuroticism and conscientiousness, and daily affect quality using full maximum likelihood estimation in HLM (N = 104). Effect (se)
Model 1: main effects
Fixed effects estimates Intercept CES-D Hormonal contraceptives Smoking Body mass index Physical activity Sleep duration Wake-up time Sex Neuroticism Conscientiousness Positive affect Negative affect Positive affect neuroticism Positive affect conscientiousness Random effects estimates Residual variance Intercept lv 1 variance Intercept lv 2 variance Deviance reduction (based on FML; (df) * **
6683.06** (269.33) 520.49 (443.47) 128.38 (431.52) 293.28 (235.01) 122.05* (55.51) 169.55* (81.91) 1.07 (3.49) 7.13 (4.82) 148.95 (352.32) 760.00* (313.31) 58.17 (263.46)
4,819,636.24 662,801.67 715,532.77 20.87* (10)
Model 2: interaction effects 6690.23** (271.96) 494.10 (441.10) 208.19 (430.98) 294.05 (235.00) 123.02* (56.76) 164.18* (81.04) .62 (3.48) 6.71 (4.80) 182.82 (364.40) 755.59* (320.52) 69.39 (262.90) 2184.94** (452.43) 313.68 (445.20) 611.34 (578.34) 1402.68* (638.22) 3,999,483.25 825,703.77 681,411.20 129.09** (16)
p < .05. p < .01.
indicate that N was positively associated with higher mean daily AUCs whereas C was unrelated to mean daily AUCs. This means that participants with high N scores displayed elevated daily cortisol levels as compared to individuals with low N scores. The model explained 6.5% of the variance in daily cortisol levels.
mean daily positive affect and lower mean daily negative affect than individuals with low C. Hence, the two personality characteristics were meaningfully related to daily affect and showed distinct associations with daily affect. Model 1 explained 10.9% of the variance in daily positive affect. Model 2 explained 18.04% of the variance in daily negative affect.
3.2. Personality and daily affect 3.3. Personality, daily affect, and cortisol levels We next estimated the relationship between N and C and daily affect (Table 2, Models 1 and 2). Results show that individuals scoring high on N reported lower mean daily positive affect and higher mean daily negative affect than individuals low in N. Individuals with high C, reported higher
A final aim of this study was to investigate more complex associations between personality, daily affect, and cortisol levels. Specifically, we wanted to know whether personality interacted with daily affect to predict daily AUCs. Results are
Table 2 Hierarchical linear models predicting daily affect quality from neuroticism and conscientiousness using full maximum likelihood estimation in HLM (N = 104). Effect (se) Fixed effects estimates Intercept Sex Neuroticism Conscientiousness Random effects estimates Residual variance Intercept lv 1 variance Intercept lv 2 variance Deviance reduction (based on FML; (df) * **
p < .05. p < .01.
Model 1: positive affect 3.48** (.05) .03 (.06) .22** (.05) .15* (.07) .15 .06 .04 23.84** (3)
Model 2: negative affect 1.88** (.06) .05 (.06) .22** (.08) .32** (.09) .11 .09 .07 30.25** (3)
1576
Figure 1 Interaction of conscientiousness and positive affect on cortisol area under the curve (AUC).
shown in Table 1 (Model 2). Our results indicate again that N was positively associated with mean daily AUCs. In addition, we also found within-person associations between daily positive affect and daily AUCs. On days when participants reported higher positive affect they where more likely to display lower AUCs as compared to days characterized by lower positive affect. No such association emerged for daily negative affect. Importantly, we further detected a significant interaction between C and daily positive affect on daily AUCs. Hence, the effect of positive affect on daily AUCs was stronger for individuals high in C. This interaction is illustrated Fig. 1 by contrasting positive affect-AUC associations in individuals high in C (1 SD above the mean) with individuals low in C (1 SD below the mean). No interaction was observed regarding C and negative affect or N and daily affect quality. Model 2 explained 13.65% of the variance in daily cortisol secretion.
4. Discussion We found that higher scores in N were associated with higher levels of cortisol throughout the measurement period, whereas no main effect of C on daily cortisol was found. When accounting for daily fluctuations in positive and negative affect, we found that individuals with high scores on C displayed reductions in daily cortisol concentrations that were driven by positive affect compared to individuals with low C scores. No such association emerged for N. We were thus able to show that N exerts a direct effect on daily cortisol levels, whereas C moderates the association between daily positive affect and cortisol. Our results are in accordance with other studies reporting on a positive relationship between N and basal cortisol (Portella et al., 2005; Wetherell et al., 2006). It needs to be critically noted, though, that these studies employed only one measurement time point and/or only one collection day. We present here the first study in which basal cortisol has been sampled over an extended period of time. Our results in adults complement previous findings of a study in adolescents using a similar assessment method but different cortisol indices (Hauner et al., 2008). Specifically, these authors found a negative association between N and daily cortisol slopes (in male participants only). The positive association in our study
U.M. Nater et al. might explain to some extent how high N might be associated with negative health outcomes (Lahey, 2009): N is suggested to exert its effect via increased basal HPA axis activity (as indicated by high cortisol levels over a certain period of time), which might lead to allostatic load and, ultimately, to dysfunction in metabolic, immune, and cardiovascular systems (Seeman and Gruenewald, 2006; Chrousos, 2009). To the best of our knowledge, no study so far has examined the relationship between C and daily cortisol. Importantly, the association between C and cortisol was moderated by positive affect in our study. This finding bears important consequences for a better understanding of how high scores in C translate into positive health outcomes, as a variety of studies have shown (Kern and Friedman, 2008). It might be proposed that C exerts its positive influence on health via affect-related regulation of HPA axis activity. In particular, it could be hypothesized that individuals with high scores in C tend to be more organized regarding their daily life matters and are thus less likely to experience distress by challenging everyday life tasks. Our findings need to be considered in the light of a few limitations. First, we have examined associations of personality and basal cortisol. A variety of studies have used dynamic measurement of cortisol by using pharmacological (McCleery et al., 2000; Zobel et al., 2004) or psychological (Phillips et al., 2005; Oswald et al., 2006) challenge tests. Considering that the effects of N in particular might be most pronounced in a situation of stress reactivity, future studies should emphasize measurement of dynamic tests and implement assessment of affect. Second, the effect of N and C on health outcomes might not only be moderated by affect, but also by a range of health behaviors (Hagger-Johnson and Whiteman, 2007). However, we controlled for a spectrum of health behaviors, such as exercise, sleep, and smoking in the current study, and our findings were not affected by these behaviors. Third, our study is based on 1-week time-sampling information that captures multiple interrelated processes. As a result, causal inferences are not allowed. Experimental research would complement our approach by testing the respective mechanisms under controlled laboratory conditions. Fourth, we did not control for effects of food intake on cortisol levels. Future studies will need to incorporate measurement of food intake in order to control for this potential confound. Finally, effect sizes are relatively small. Although this is true for most time-sampling research, it is still important to point out that the data capture a multitude of complex daily life processes that are difficult to control for (Hoppmann and Riediger, 2009). In conclusion, the findings of this study have the potential to further elucidate the role of personality in HPA axis regulation and, thus, improve our understanding of the association of endocrine states and health outcomes.
Role of the funding sources The funding agency had no role in data collection, data analysis, or writing of the manuscript.
Conflict of Interest The authors declare no conflict of interest.
Personality and daily cortisol Disclosure Information: The authors declare no financial interest related to the study.
Acknowledgments The research reported here was funded by a Volkswagen Foundation grant to Petra Klumb.
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