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Personality and Individual Differences 43 (2007) 865–872

Directional asymmetry (right–left differences) indigit ratio (2D:4D) predict indirect aggression in women

Sarah M. Coyne *, John T. Manning, Leanne Ringer, Lisa Bailey

Department of Psychology, University of Central Lancashire, Preston, PR1 2HE Lancashire, United Kingdom

Received 11 October 2006; received in revised form 16 January 2007; accepted 2 February 2007Available online 21 March 2007

Abstract

A large body of research has revealed that digit length ratios (2D:4D) are influenced by exposure to pre-natal androgens. It is thought that higher exposure to prenatal androgens leads to the development of moremasculinized (smaller) digit ratios. Low 2D:4D, particularly low right 2D:4D and low right–left 2D:4D(directional asymmetry or DA) has been linked with a number of behavioral traits which are sex-dependent,including performance in sports and exercise, and to some degree, aggression. To date, the focus of digitratio research has been on physical aggression, however, 2D:4D has never been linked with indirect aggres-sion (also called social or relational aggression), a form of aggression often preferred by women. We mea-sured the 2D:4D of 100 women and compared these scores with responses on indirect and direct aggressionquestionnaires. Although 2D:4D was not linked to direct aggression in women, we found that low DA pre-dicted indirect aggression. We conclude that higher levels of prenatal testosterone induce higher levels ofaggression, and that the link between prenatal testosterone and aggression in women is most strongly seenfor indirect aggression.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Indirect aggression; Relational aggression; Social aggression; Testosterone; Digit ratio; 2D:4D; Directionalasymmetry; DA

0191-8869/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.paid.2007.02.010

* Corresponding author. Fax: +44 1772892925.E-mail address: smcoyne@uclan.ac.uk (S.M. Coyne).

866 S.M. Coyne et al. / Personality and Individual Differences 43 (2007) 865–872

1. Introduction

The ratio of the length of the 2nd (index) and 4th (ring) fingers (digit ratio or 2D:4D) may benegatively correlated with prenatal testosterone (PT). Thus, 2D:4D is sexually dimorphic (withlower values in males compared to females) in children and adults (Manning, Scutt, Wilson, &Lewis-Jones, 1998). This dimorphism appears in the fetus (Malas, Dogan, Hilal Evcil, & Desdi-cioglu, 2006), and is little affected by puberty (McIntyre, Ellison, Lieberman, Demerath, &Towne, 2005; Trivers, Manning, & Jacobsen, 2006). Prenatal testosterone may be causal forthe sexual dimorphism because (i) children with congenital adrenal hyperplasia (CAH), a traitassociated with high prenatal androgen, have lower 2D:4D than controls (Brown, Hines, Fane,& Breedlove, 2002; Okten, Kalyoncu, & Yaris, 2002; but see Buck, Williams, Hughes, & Acerini,2003); (ii) females with CAH often show ovarian hyperandrogenism which has similarities to poly-cystic ovary syndrome (PCOS) and patients with PCOS have lower 2D:4D than controls (Cattrall,Vollenhoven, & Weston, 2005); (iii) analyses of routine amniocentesis samples show a negativerelationship between testosterone:oestrogen ratios and 2D:4D of children (Lutchmaya, Baron-Cohen, Raggatt, Knickmeyer, & Manning, 2004); (iv) maternal smoking during pregnancy ele-vates PT and is associated with a reduction in children’s 2D:4D (Rizwan, Manning, & Brabin,2006).

The tendency for males to show more physical aggression than females is one of the most wellestablished sex differences in the psychological literature (e.g. Buss, 1961). The link between tes-tosterone and aggression is also well established, although, there is some debate surrounding thestrength of the association (e.g. Archer, 2006; Hines, 2004).

To our knowledge, only three studies have examined the link between aggression and digit ra-tio. Austin, Manning, McInroy, and Mathews (2002) considered whether digit ratio correlatedwith various subscales on Buss and Perry’s (1992) aggression questionnaire. This questionnaireinvolves four subscales, namely, anger, hostility, verbal aggression, and physical aggression. Aus-tin et al. (2002) found that digit ratio was not linked with any of the aggression subscales for menor women, meaning that prenatal testosterone may not have an influence on aggression.

Bailey and Hurd (2005) attempted to replicate Austin et al.’s (2002) findings, also using Buss andPerry’s (1992) aggression questionnaire. They confirmed Austin et al.’s (2002) results with women,finding no relationship between aggression and digit ratio. This was not surprising, given the lowrate at which women physically aggress. However, they did find that digit ratio predicted physicalaggression (but not other subscales) for men. Specifically, those men with more masculinized digitratios (and therefore exposed to higher levels of prenatal testosterone) reported being more phys-ically aggressive than other men. As a whole, Bailey and Hurd’s (2005) results are consistent withthe view that testosterone has an organizational influence on physical aggression in men.

Benderlioglu and Nelson (2004) also examined the link between digit ratio and aggression, how-ever, they focussed on reactive aggression a type of aggression that involves an angry outburst tofrustration or provocation. Unlike proactive aggression, reactive aggression is not pre-meditated,rather it is an immediate response to something unpleasant. To induce aggression, participantswere asked to raise money for a fictitious charity organization. They made several calls to confed-erates of the experiment who were either kind, but not willing to donate, or hostile. Reactiveaggression was measured by how forceful participants put down the telephone, and the toneof a prepared follow-up letter to the confederate. Results revealed that digit ratio was related

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to reactive aggression, but only in women. This shows that certain types of aggression in womenmay be influenced by prenatal testosterone.

Combined, these studies show mixed results: namely that digit ratio is sometimes related tophysical aggression in men, but reactive aggression in women (which can be physical). One formof aggression not mentioned in either study is indirect aggression. This type of aggression is amanipulative, often covert form of aggression where the aim is social exclusion or reputationalmanipulation (see Archer & Coyne, 2005). Examples of indirect aggression include spreading ru-mors, social exclusion, breaking confidences, dirty looks, etc. Indirect aggression has also beencalled relational (e.g. Crick & Grotpeter, 1995), and social aggression (e.g. Underwood, 2003),however, we refer to such behavior as ‘‘indirect aggression’’ in this paper as the term is widespreadand has precedence over the other two terms (Archer & Coyne, 2005). Indirect aggression can bereactive, but is usually proactive, with the aggressor actively planning their aggression. The reac-tive aggression in Benderlioglu and Nelson’s (2004) study was covert, but was not a good exampleof indirect aggression as the purpose was neither to socially exclude nor to manipulate another’sreputation. Rather, the purpose of the aggression seemed to be to release frustration.

Research has shown that females generally use more indirect aggression than males, however,this varies by age and methodology utilized (Archer, 2004; Hyde, 2005). Sex differences are great-est in adolescence, when individuals have the necessary verbal and social skills needed to success-fully use this type of aggression (e.g. Archer, 2004; Bjorkqvist, Lagerspetz, & Kaukiainen, 1992).Society often frowns upon female aggression, as females are supposed to be ‘‘sugar, and spice, andall that’s nice’’. There are less social sanctions against using indirect aggression as the aggressorcan often remain anonymous. Females also highly value relationships and social status, therefore,indirect aggression may be a particularly effective way to harm another member of the group. Byadulthood, the use of indirect aggression is more equal (Archer, 2004), however, some studieshave found that females use more indirect aggression than males (e.g. Bjorkqvist, Osterman, &Lagerspetz, 1994).

Although hidden and disguised, indirect aggression is still a form of aggression. Therefore, itmay be subject to the same organizational influences of testosterone as physical aggression.Although the role of testosterone has been well researched in the field of physical aggression, ithas never been examined in the context of indirect aggression. Therefore, the purpose of the cur-rent study is to examine whether 2D:4D correlates with indirect aggression in women. We hypoth-esize that individuals with high PT (more masculinized digit ratios) will be more aggressive ingeneral. However, because men and women prefer to express aggression in different forms we pre-dicted that in women, masculinized digit ratios will be related to indirect rather than direct aggres-sion. There is evidence that right 2D:4D and right–left 2D:4D, termed directional asymmetry(DA) appear to be stronger correlates of prenatal testosterone than left 2D:4D (Bailey & Hurd,2005; Manning, 2002; Manning et al., 1998). We predicted relationships between 2D:4D andaggression will be strongest for right 2D:4D and DA. For consistency, we also measured direct(physical) aggression. As previous research reveals that direct aggression does not link with2D:4D in women (e.g. Bailey & Hurd, 2005), we hypothesized that there will be no relationshipbetween the two. We have decided to examine the relationship between digit ratio and indirectaggression in women initially, as women generally prefer this form of aggression over other types(e.g. Richardson & Green, 1999). Males, on the other hand, generally prefer the use of physicalaggression, although this does depend on the context examined.

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2. Method

2.1. Participants

One-hundred females took part in the study. The sample consisted of undergraduate studentsattending a large sized University in the North West of England. The age of participants rangedfrom 18 to 33 years old, with a mean age of 21.07 years (SD = 2.33).

2.2. Procedure

Participants taking part in the study completed a series of questionnaires (see below) and their2nd and 4th fingers were measured. The study took place in a quiet laboratory room in the psy-chology department.

2.3. Measures

The aggression questionnaire (Buss & Perry, 1992) was used as a measure of direct aggression.This questionnaire is likely the most commonly used measure of self-report aggression and asksparticipants to read a variety of statements and mark down how characteristic each statement is oftheir behavior on a 5-point Likert scale (1 = extremely uncharacteristic, 5 = extremely character-istic). The questionnaire consisted of four subscales, namely, physical aggression (nine items), ver-bal aggression (five items), anger (seven items), and hostility (eight items). An overall mean scorewas also calculated to give each person a composite direct aggression score (DiAg). As in otherstudies, all subscales and composite measures showed good internal reliability (PA: a = .78;VA: a = .75; A: a = .73; H: a = .76; DiAg: a = .88).

The indirect aggression questionnaire (Forrest, Eatough, & Shevlin, 2005) was used as a mea-sure of indirect aggression. This scale is relatively new, and again asks participants to read a vari-ety of statements and mark down the degree (on a 5-point Likert scale) to which each ischaracteristic of their behavior (1 = never, 5 = regularly). However, unlike the AQ, this question-naire asks participants to only rate behavior occurring in the previous 12 months. Three differentforms of indirect aggression are measured on the IAQ. Social exclusion (SE) is behavior that ex-cludes other people from a group or conversation. This subscale had ten items and consisted ofstatements such as ‘‘spread rumors about them’’, and ‘‘excluded them from a group’’. The mali-cious humour (MH) subscale measured behavior that was ‘‘rational appearing’’, that seemedinnocent or harmless if questioned. This subscale involved nine items, and included statementssuch as ‘‘imitated them in front of others’’, and ‘‘played a nasty practical joke on them’’. Finally,the guilt induction (GI) subscale measured behavior that played on a person’s guilt or emotionalstate. This scale had six items and consisted of statements including ‘‘tried to influence them bymaking them feel guilty’’, and ‘‘used their feelings to coerce them’’. An overall mean of these sub-scales was calculated to give each person a composite indirect aggression score (InAg). As in theoriginal Forrest et al. (2005) paper, the subscales and composite score had good internal reliability(SE: a = .90; MH: a = .89; GI: a = .84; InAg: a = .94).

Participants’ 2nd and 4th digit lengths were measured (to the nearest .01 mm) using adigital calliper. We decided to measure digit length directly from the fingers as opposed to using

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photocopies of the fingers which tend to be less reliable (Manning, Fink, Neave, & Caswell, 2005).Digits were measured on the ventral surface of the hand, from the basal crease of the digit to thetip. Injuries were noted, and broken digits or digits with injuries that occurred within the previoussix months were omitted from the sample. Each measurement was taken twice (first the right handand then the left, and then repeated) to assess reliability. The mean of these two measurementswas taken and the ratio between the 2nd and 4th digit was calculated. A high positive correlationwas found between the left digit ratio and the right digit ratio (r = .61, p < .001) a figure that cor-responds well with other research in this field (Voracek, Manning, & Ponocny, 2005). Finally, theleft digit ratio was subtracted from the right digit ratio to give a measure of directional asymmetryof 2D:4D. A negative figure indicated a more masculinized pattern of digit ratio in the right hand,while a positive figure indicated the opposite. This measure of directional asymmetry (DA) wasused in subsequent analyses.

3. Results

Data screening revealed that all of the indirect aggression scales were positively skewed. Con-sistent with Forrest et al. (2005) we transformed the data for each scale into its logarithm.

Age was positively correlated with right hand 2D:4D (right 2D:4D, r = .26, p < .001; left 2D:4Dr = .09, p = .17) and DA (r = .17, p < .01) and was controlled for in each subsequent analysis asage has been found to have a mild influence on 2D:4D (e.g. Trivers et al., 2006). There were nosignificant relationships between 2D:4D and total direct aggression (right 2D:4D, r = �.17,p = .09; left 2D:4D r = �.04, p = .72) or total indirect aggression (right 2D:4D, r = �.13,p = .21, left 2D:4D, r = .13, p = .21). However, DA was significantly negatively correlated withall three indirect aggression subscales, namely social exclusion (r = �.23, p = .02), malicious hu-mour (r = �.24, p = .02), and guilt induction (r = �.31, p = .004). A significant correlation wasalso found between DA and the total indirect aggression scale (r = �.30, p = .004). Significantcorrelations were not found between DA and any of the direct aggression (AQ) subscales.

A series of hierarchical regression analyses were performed for each subscale and compositescore. Table 1 shows a summary of the results for each variable. DA did not predict any formof direct aggression. Indirect aggression (and each related subscale) was correlated with DA in

Table 1D r � l and type of aggression

B b t p r2 (adj.) Overall ANOVA results

Direct aggression composite �.007 �.15 1.49 .14 .02 F(2,96) = 1.67, p = .19Anger �.002 �.06 .58 .56 .00 F(2,96) = .72, p = .49Hostility �.004 �.11 1.05 .30 .01 F(2,96) = 1.11, p = .33Physical aggression �.006 �.19 1.88 .06 .03 F(2,96) = 2.34, p = .10Verbal aggression �.003 �.08 .78 .44 .00 F(2,96) = .85, p = .43Indirect aggression composite �.03 �.30 3.09 .003 .09 F(2,96) = 5.39, p < .01Guilt induction �.02 �.30 3.11 .003 .09 F(2,96) = 5.43, p < .01Malicious humuor �.02 �.24 2.36 .02 .06 F(2,96) = 3.36, p < .05Social exclusion �.02 �.23 2.33 .02 .05 F(2,96) = 3.31, p < .05

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the direction predicted. Namely, more masculine (lower) DA predicted all three types of indirectaggression in women. Essentially, this means that those women who are high in prenatal testos-terone tend to use indirect strategies to aggress against others.

4. Discussion

We found that lower values of DA predicted all types of indirect aggression in women. Specif-ically, we found that women who had low DA are likely to be more indirectly aggressive than wo-men who had high DA. There is accumulating evidence that PT influences right 2D:4D and DAmore than left 2D:4D (Cattrall et al., 2005; Lutchmaya et al., 2004; Manning, 2002; Manninget al., 1998, p. 21; Rizwan et al., 2006). Benderlioglu and Nelson (2004) found that testosteronelevels in men correlated negatively with right 2D:4D and DA but not with left 2D:4D. In additionlow right 2D:4D and low DA were related to high reactive aggression. Intriguingly there appearsto be a more intense expression of ‘‘masculine’’ traits on the right side of the body than on the left,and the lateralised expression of 2D:4D is consistent with this pattern (Manning, 2002, p. 21).

We think the association between DA and aggression means that female aggression is also sub-ject to testosterone’s organizational influence. However, it appears that testosterone may increaseaggression in general but the expression of the aggression may be gender specific, with high PTshowing a link with physical aggression in men, and only indirect aggression in women. Thisexplanation is supported by other research, including the study of Benderlioglu and Nelson(2004) who measured a reactive covert form of aggression that was similar to indirect aggression.Their study found that DA was related to this type of aggression in women, but not in men. Otherresearch has found that digit ratio is related to physical aggression in men, but not in women (Bai-ley & Hurd, 2005). The current results, therefore, provide evidence of a biological explanation forthe sex difference in aggression. Although the use of indirect aggression is clearly influenced bysocietal constraints and expectations, this study shows that the choice of aggression by menand women may be partially biological in nature.

This study also shows that indirect and physical aggression have similarities. Although theymay appear different in form and in motivation, the primary purpose of both forms of aggressionis to hurt another human being. Both forms of aggression are influenced by prenatal testosterone,and in the same direction. Although this is expressed differentially in men and women, this studyshows that the two forms of aggression are similar at even a biological level.

These results support Benderlioglu and Nelson’s (2004) findings that prenatal testosterone doesinfluence certain types of aggression in women. They found that females with more masculinizeddigit ratios were more reactively aggressive toward a confederate than other females, specificallyslamming a phone down more forcefully, and writing more aggressive follow-up letters. As statedearlier, these forms of aggression are covert, and may therefore be more appealing to women thanother forms of aggression. However, these forms of aggression are somewhat removed from theclassic forms of indirect aggression that are typically measured. When using a measure containingtraditional forms of indirect aggression (e.g. spreading rumors, social exclusion, etc.), the currentstudy still found that more masculinized digit ratios predicted these forms of indirect aggression.

In summary, this study has provided the first evidence that prenatal hormones influence the useof indirect aggression. Similar to physical aggression, indirect aggression appears to be subject to

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testosterone’s organizational influence, however, this influence leads to a gender specific expres-sion of aggression. Our results add one more piece to the puzzle to explain why individuals useindirect forms of aggression to hurt those around them.

References

Archer, J. (2004). Sex differences in aggression in real-world settings: a meta-analytic review. Review of General

Psychology, 8, 291–332.Archer, J. (2006). Testosterone and human aggression: an evaluation of the challenge hypothesis. Neuroscience and

Biobehavioral Reviews, 30, 319–345.Archer, J., & Coyne, S. M. (2005). An integrated review of indirect, relational and social aggression. Personality and

Social Psychology Review, 9, 212–230.Austin, E. J., Manning, J. T., McInroy, K., & Mathews, E. (2002). An investigation of the associations between

personality, cognitive ability, and digit ratio. Personality and Individual Differences, 33, 1115–1124.Bailey, A., & Hurd, P. L. (2005). Finger length ratio (2D:4D) correlates with physical aggression in men, but not

women. Biological Psychology, 68, 215–222.Benderlioglu, Z., & Nelson, R. J. (2004). Digit length ratio predict reactive aggression in women, but not men.

Hormones and Behaviour, 46, 558–564.Bjorkqvist, K., Lagerspetz, K., & Kaukiainen, A. (1992). Do girls manipulate and boys fight? Developmental trends in

regard to direct and indirect aggression. Aggressive Behavior, 18, 117–127.Bjorkqvist, K., Osterman, K., & Lagerspetz, K. (1994). Sex differences in covert aggression among adults. Aggressive

Behavior, 20, 27–33.Brown, W. M., Hines, M., Fane, B. A., & Breedlove, S. M. (2002). Masculinized finger length patterns in human males

and females with congenital adrenal hyperplasia. Hormones and Behavior, 42, 380–386.Buck, J. J., Williams, R. M., Hughes, I. A., & Acerini, C. L. (2003). In-utero androgen exposure and 2nd to 4th digit

length ratio-comparisons between healthy controls and females with classical congenital adrenal hyperplasia. Human

Reproduction, 18, 976–979.Buss, A. (1961). The psychology of aggression. New York: John Wiley and Sons.Buss, A., & Perry, M. (1992). The aggression questionnaire. Journal of Personality and Social Psychology, 63, 452–459.Cattrall, F. R., Vollenhoven, B. J., & Weston, G. C. (2005). Anatomical evidence for in utero androgen exposure in

women with polycystic ovary syndrome. Fertility and Sterility, 84, 1689–1692.Crick, N. R., & Grotpeter, J. (1995). Relational aggression, gender, and social-psychological adjustment. Child

Development, 66, 710–722.Forrest, S., Eatough, V., & Shevlin, M. (2005). Measuring adult indirect aggression: the development and psychometric

assessment of the indirect aggression scales. Aggressive Behavior, 31, 84–97.Hines, M. (2004). Androgen, estrogen and gender: contribution of the early hormone environment to gender related

behavior. In A. H. Eagly, A. E Beall, & R. J. Sternberg (Eds.), The psychology of gender (pp. 9–37). New York:Guilford Press.

Hyde, J. S. (2005). The gender similarity hypothesis. American Psychologist, 60, 581–592.Lutchmaya, S., Baron-Cohen, S., Raggatt, P., Knickmeyer, R., & Manning, J. T. (2004). Maternal 2nd to 4th digit

ratios and foetal testosterone. Early Human Development, 77, 23–28.Malas, M. A., Dogan, S., Hilal Evcil, E., & Desdicioglu, K. (2006). Fetal development of the hand, digits and digit ratio

(2D:4D). Early Human Development, 82, 469–475.Manning, J. T. (2002). Digit ratio: A pointer to fertility, behaviour and health. New Brunswick: Rutgers University Press.Manning, J., Fink, B., Neave, N., & Caswell, N. (2005). Photocopies yield lower digit ratios (2D:4D) than direct finger

measurements. Archives of Sexual Behavior, 34, 329–333.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.

872 S.M. Coyne et al. / Personality and Individual Differences 43 (2007) 865–872

McIntyre, M. H., Ellison, P. T., Lieberman, D. E., Demerath, E., & Towne, B. (2005). The development of sexdifferences in digital formula from infancy in the Fels Longitudinal Study. Proceedings of the Biological Sciences,

272, 1473–1479.Okten, A., Kalyoncu, M., & Yaris, N. (2002). The ratio of second- and fourth-digit lengths and congenital adrenal

hyperplasia due to 21-hydroxylase deficiency. Early Human Development, 70, 47–54.Richardson, D. R., & Green, L. R. (1999). Social sanction and threat explanations of gender effects on direct and

indirect aggression. Aggressive Behavior, 25, 425–434.Rizwan, S., Manning, J. T., & Brabin, B. J. (2006). Maternal smoking during pregnancy and possible effects of in utero

testosterone: evidence from the 2D:4D finger length ratio. Early Human Development [Epub ahead of print].Trivers, R., Manning, J., & Jacobsen, A. (2006). A longitudinal study of digit ratio (2D:4D) and other finger ratios in

Jamaican children. Hormones and Behavior, 49, 150–156.Underwood, M. K. (2003). Social aggression among girls. New York: Guilford Press.Voracek, M., Manning, J. T., & Ponocny, I. (2005). Digit ratio (2D:4D) in homosexual and heterosexual men from

Austria. Archives of Sexual Behavior, 34, 335–340.