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

Transcription

1 Personality and Individual Differences 43 (2007) Directional asymmetry (right left differences) in digit 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 2007 Available online 21 March 2007 Abstract A large body of research has revealed that digit length ratios (2D:4D) are influenced by exposure to prenatal androgens. It is thought that higher exposure to prenatal androgens leads to the development of more masculinized (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 digit ratio research has been on physical aggression, however, 2D:4D has never been linked with indirect aggression (also called social or relational aggression), a form of aggression often preferred by women. We measured the 2D:4D of 100 women and compared these scores with responses on indirect and direct aggression questionnaires. Although 2D:4D was not linked to direct aggression in women, we found that low DA predicted indirect aggression. We conclude that higher levels of prenatal testosterone induce higher levels of aggression, and that the link between prenatal testosterone and aggression in women is most strongly seen for indirect aggression. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Indirect aggression; Relational aggression; Social aggression; Testosterone; Digit ratio; 2D:4D; Directional asymmetry; DA * Corresponding author. Fax: address: smcoyne@uclan.ac.uk (S.M. Coyne) /$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi: /j.paid

2 866 S.M. Coyne et al. / Personality and Individual Differences 43 (2007) Introduction The ratio of the length of the 2nd (index) and 4th (ring) fingers (digit ratio or 2D:4D) may be negatively correlated with prenatal testosterone (PT). Thus, 2D:4D is sexually dimorphic (with lower 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, & Desdicioglu, 2006), and is little affected by puberty (McIntyre, Ellison, Lieberman, Demerath, & Towne, 2005; Trivers, Manning, & Jacobsen, 2006). Prenatal testosterone may be causal for the sexual dimorphism because (i) children with congenital adrenal hyperplasia (CAH), a trait associated 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 polycystic 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 negative relationship between testosterone:oestrogen ratios and 2D:4D of children (Lutchmaya, Baron- Cohen, Raggatt, Knickmeyer, & Manning, 2004); (iv) maternal smoking during pregnancy elevates 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 well established sex differences in the psychological literature (e.g. Buss, 1961). The link between testosterone and aggression is also well established, although, there is some debate surrounding the strength of the association (e.g. Archer, 2006; Hines, 2004). To our knowledge, only three studies have examined the link between aggression and digit ratio. Austin, Manning, McInroy, and Mathews (2002) considered whether digit ratio correlated with various subscales on Buss and Perry s (1992) aggression questionnaire. This questionnaire involves four subscales, namely, anger, hostility, verbal aggression, and physical aggression. Austin et al. (2002) found that digit ratio was not linked with any of the aggression subscales for men or 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 and Perry 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 low rate at which women physically aggress. However, they did find that digit ratio predicted physical aggression (but not other subscales) for men. Specifically, those men with more masculinized digit ratios (and therefore exposed to higher levels of prenatal testosterone) reported being more physically aggressive than other men. As a whole, Bailey and Hurd s (2005) results are consistent with the 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, however, they focussed on reactive aggression a type of aggression that involves an angry outburst to frustration or provocation. Unlike proactive aggression, reactive aggression is not pre-meditated, rather it is an immediate response to something unpleasant. To induce aggression, participants were asked to raise money for a fictitious charity organization. They made several calls to confederates of the experiment who were either kind, but not willing to donate, or hostile. Reactive aggression was measured by how forceful participants put down the telephone, and the tone of a prepared follow-up letter to the confederate. Results revealed that digit ratio was related

3 S.M. Coyne et al. / Personality and Individual Differences 43 (2007) to reactive aggression, but only in women. This shows that certain types of aggression in women may be influenced by prenatal testosterone. Combined, these studies show mixed results: namely that digit ratio is sometimes related to physical aggression in men, but reactive aggression in women (which can be physical). One form of aggression not mentioned in either study is indirect aggression. This type of aggression is a manipulative, often covert form of aggression where the aim is social exclusion or reputational manipulation (see Archer & Coyne, 2005). Examples of indirect aggression include spreading rumors, social exclusion, breaking confidences, dirty looks, etc. Indirect aggression has also been called 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 widespread and has precedence over the other two terms (Archer & Coyne, 2005). Indirect aggression can be reactive, but is usually proactive, with the aggressor actively planning their aggression. The reactive aggression in Benderlioglu and Nelson s (2004) study was covert, but was not a good example of indirect aggression as the purpose was neither to socially exclude nor to manipulate another s reputation. 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 greatest in adolescence, when individuals have the necessary verbal and social skills needed to successfully use this type of aggression (e.g. Archer, 2004; Björkqvist, Lagerspetz, & Kaukiainen, 1992). Society often frowns upon female aggression, as females are supposed to be sugar, and spice, and all that s nice. There are less social sanctions against using indirect aggression as the aggressor can 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. By adulthood, the use of indirect aggression is more equal (Archer, 2004), however, some studies have found that females use more indirect aggression than males (e.g. Björkqvist, Österman, & Lagerspetz, 1994). Although hidden and disguised, indirect aggression is still a form of aggression. Therefore, it may 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, it has never been examined in the context of indirect aggression. Therefore, the purpose of the current study is to examine whether 2D:4D correlates with indirect aggression in women. We hypothesize that individuals with high PT (more masculinized digit ratios) will be more aggressive in general. However, because men and women prefer to express aggression in different forms we predicted that in women, masculinized digit ratios will be related to indirect rather than direct aggression. 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 and aggression 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 with 2D:4D in women (e.g. Bailey & Hurd, 2005), we hypothesized that there will be no relationship between the two. We have decided to examine the relationship between digit ratio and indirect aggression 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 physical aggression, although this does depend on the context examined.

4 868 S.M. Coyne et al. / Personality and Individual Differences 43 (2007) Method 2.1. Participants One-hundred females took part in the study. The sample consisted of undergraduate students attending a large sized University in the North West of England. The age of participants ranged from 18 to 33 years old, with a mean age of years (SD = 2.33) Procedure Participants taking part in the study completed a series of questionnaires (see below) and their 2nd and 4th fingers were measured. The study took place in a quiet laboratory room in the psychology department 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 asks participants to read a variety of statements and mark down how characteristic each statement is of their behavior on a 5-point Likert scale (1 = extremely uncharacteristic, 5 = extremely characteristic). The questionnaire consisted of four subscales, namely, physical aggression (nine items), verbal aggression (five items), anger (seven items), and hostility (eight items). An overall mean score was also calculated to give each person a composite direct aggression score (DiAg). As in other studies, 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 measure of indirect aggression. This scale is relatively new, and again asks participants to read a variety of statements and mark down the degree (on a 5-point Likert scale) to which each is characteristic of their behavior (1 = never, 5 = regularly). However, unlike the AQ, this questionnaire asks participants to only rate behavior occurring in the previous 12 months. Three different forms of indirect aggression are measured on the IAQ. Social exclusion (SE) is behavior that excludes other people from a group or conversation. This subscale had ten items and consisted of statements such as spread rumors about them, and excluded them from a group. The malicious humour (MH) subscale measured behavior that was rational appearing, that seemed innocent or harmless if questioned. This subscale involved nine items, and included statements such 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 emotional state. This scale had six items and consisted of statements including tried to influence them by making them feel guilty, and used their feelings to coerce them. An overall mean of these subscales was calculated to give each person a composite indirect aggression score (InAg). As in the original 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 a digital calliper. We decided to measure digit length directly from the fingers as opposed to using

5 S.M. Coyne et al. / Personality and Individual Differences 43 (2007) 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 the tip. Injuries were noted, and broken digits or digits with injuries that occurred within the previous six months were omitted from the sample. Each measurement was taken twice (first the right hand and then the left, and then repeated) to assess reliability. The mean of these two measurements was taken and the ratio between the 2nd and 4th digit was calculated. A high positive correlation was found between the left digit ratio and the right digit ratio (r =.61, p <.001) a figure that corresponds well with other research in this field (Voracek, Manning, & Ponocny, 2005). Finally, the left digit ratio was subtracted from the right digit ratio to give a measure of directional asymmetry of 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) was used in subsequent analyses. 3. Results Data screening revealed that all of the indirect aggression scales were positively skewed. Consistent 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:4D r =.09, p =.17) and DA (r =.17, p <.01) and was controlled for in each subsequent analysis as age has been found to have a mild influence on 2D:4D (e.g. Trivers et al., 2006). There were no significant 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 with all three indirect aggression subscales, namely social exclusion (r =.23, p =.02), malicious humour (r =.24, p =.02), and guilt induction (r =.31, p =.004). A significant correlation was also found between DA and the total indirect aggression scale (r =.30, p =.004). Significant correlations 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 composite score. Table 1 shows a summary of the results for each variable. DA did not predict any form of direct aggression. Indirect aggression (and each related subscale) was correlated with DA in Table 1 D r l and type of aggression B b t p r 2 (adj.) Overall ANOVA results Direct aggression composite F(2,96) = 1.67, p =.19 Anger F(2,96) =.72, p =.49 Hostility F(2,96) = 1.11, p =.33 Physical aggression F(2,96) = 2.34, p =.10 Verbal aggression F(2,96) =.85, p =.43 Indirect aggression composite F(2,96) = 5.39, p <.01 Guilt induction F(2,96) = 5.43, p <.01 Malicious humuor F(2,96) = 3.36, p <.05 Social exclusion F(2,96) = 3.31, p <.05

6 870 S.M. Coyne et al. / Personality and Individual Differences 43 (2007) the direction predicted. Namely, more masculine (lower) DA predicted all three types of indirect aggression in women. Essentially, this means that those women who are high in prenatal testosterone 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. Specifically, we found that women who had low DA are likely to be more indirectly aggressive than women who had high DA. There is accumulating evidence that PT influences right 2D:4D and DA more than left 2D:4D (Cattrall et al., 2005; Lutchmaya et al., 2004; Manning, 2002; Manning et al., 1998, p. 21; Rizwan et al., 2006). Benderlioglu and Nelson (2004) found that testosterone levels in men correlated negatively with right 2D:4D and DA but not with left 2D:4D. In addition low right 2D:4D and low DA were related to high reactive aggression. Intriguingly there appears to 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 subject to testosterone s organizational influence. However, it appears that testosterone may increase aggression in general but the expression of the aggression may be gender specific, with high PT showing a link with physical aggression in men, and only indirect aggression in women. This explanation 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. Other research has found that digit ratio is related to physical aggression in men, but not in women (Bailey & Hurd, 2005). The current results, therefore, provide evidence of a biological explanation for the sex difference in aggression. Although the use of indirect aggression is clearly influenced by societal constraints and expectations, this study shows that the choice of aggression by men and women may be partially biological in nature. This study also shows that indirect and physical aggression have similarities. Although they may appear different in form and in motivation, the primary purpose of both forms of aggression is 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 study shows 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 does influence certain types of aggression in women. They found that females with more masculinized digit ratios were more reactively aggressive toward a confederate than other females, specifically slamming a phone down more forcefully, and writing more aggressive follow-up letters. As stated earlier, these forms of aggression are covert, and may therefore be more appealing to women than other forms of aggression. However, these forms of aggression are somewhat removed from the classic forms of indirect aggression that are typically measured. When using a measure containing traditional forms of indirect aggression (e.g. spreading rumors, social exclusion, etc.), the current study 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 use of indirect aggression. Similar to physical aggression, indirect aggression appears to be subject to

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