2D:4D Ratios and FA 1 Running head: AGGRESSION, FINGER LENGTH RATIOS, AND FLUCTUATING Aggression, Finger Length Ratios, Fluctuating Asymmetry, and Verbal/Spatial Abilities in Male and Female College Students

The 2D:4D (second to fourth digit) ratio is thought to be a measure of prenatal testosterone exposure, and has been linked to traits such as aggression, sexual tendencies, and verbal/spatial intelligence. Fluctuating Asymmetry (FA) refers to deviation from bilateral symmetry caused by environmental problems during ontogeny. This study further examines the relationship between these factors. Participants completed spatial and verbal fluency tests as well as aggression and sexual history questionnaires. Morphological measures of ears, ankles, and digit lengths were taken. The results indicated that 2D:4D ratios were sexually dimorphic with males having significantly lower ratios and higher spatial and aggression scores. Pearson correlations indicated that 2D:4D ratios were negatively correlated with physical aggression. Aggression was also positively correlated with number of sexual partners. Physical aggression and Total aggression were positively associated with spatial ability for male participants, but negatively associated with spatial ability for female participants. There were no sex differences for FA scores, verbal scores, and number of sexual partners. The data are discussed in terms of prenatal testosterone exposure. 2D:4D Ratios and FA 3 Aggression, Finger Length Ratios, Fluctuating Asymmetry, and Verbal/Spatial Abilities in Male and Female College Students The 2D:4D ratio (the length of one’s index finger [2D] with respect to their ring [4D] finger) and fluctuating asymmetry (FA; variable measures for paired physical characteristics on the human body) have been correlated with spatial and verbal ability, as well as aggression and number of sexual partners. They are thought to be mediated, in part, by prenatal testosterone (T) exposure. Excessive T exposure during ontogeny can modulate adult physiology and behavior. For instance, a 2D:4D ratio of less than one is correlated with increased spatial ability (Coolican & Peters, 2002; Kempel et al., 2003), decreased verbal ability (Luxen & Buunk, 2005), aggression (Bailey & Hurd, 2005; Benderlioglue, Sciulli, & Nelson, 2004), and certain sexual behaviors (Honekopp, Voracek, & Manning, 2006; Putz, Gaulin, Sporter, & McBurney, 2004). Moreover, extreme T exposure during ontogeny has been known to correlate with congenital adrenal hyperplasia (CAH). Females with CAH have lower 2D:4D ratios than females without CAH (Brown, Hines, Fane, & Breedlove, 2001). This evidence suggests that excessive T exposure can result in low 2D:4D ratios and the previously stated physical characteristics and abilities. Our genetic makeup suggests that paired physical characteristics, such as ankle circumferences and ear lengths, should be identical in size. However, environmental and endogenous factors may lead to asymmetry (Benderlioglu, Sciulli, & Nelson, 2004). Fluctuations in the size of these paired physical characteristics can be a side effect of developmental instability, which is accredited to the organizational effects of T exposure. Reactive aggression (aggression in response to a stimulus) is positively correlated with FA in both sexes (Benderlioglu, Sciulli, & Nelson, 2004). Although T was not directly 2D:4D Ratios and FA 4 implicated, high FA can attribute to high aggression. Violent criminals (another proxy for aggression) also tend to have high FA compared to non-criminals (Lalumiere, Harris, & Rice, 2001). The use of this archival data suggests that violent criminals have more prenatal complications and FA than non-criminals. Once again, this suggests that the organizational effects of T during ontogeny can affect aggression and FA. Past literature suggests that spatial and verbal ability are partially sexually dimorphic abilities; men tend to score higher on some spatial tests and women tend to score better on some tests of verbal ability (Coolican & Peters, 2003; Luxen & Buunk, 2005; Putz et al., 2004). The 2D:4D ratio is also sexually dimorphic; men tend to have low ratios while women tend to have higher ratios. Some research suggests that women with low 2D:4D ratios score higher on spatial tests than women with higher ratios (Kempel et al., 2005). It may even be that prenatal T exposure affects the type of spatial reasoning used on specific tasks. Csatho (2001) hypothesizes that participants with high T exposure used an exploratory (using navigational cues) strategy while participants with low exposure did not. This could explain why people with low 2D:4D ratios do better on spatial tests. Aggression is also a sexually dimorphic trait that arises from excess T exposure during ontogeny (Bailey & Hurd, 2005; Benderlioglu, Sciulli, & Nelson, 2004). Masculine traits such as participating in sports, showing hostility, and showing anger are all associated with T (Benderlioglu & Nelson, 2004). Some research suggests that the 2D:4D ratio is positively correlated with number of sexual partners (Honekopp et al., 2006; Putz et al., 2004). Many studies have correlated anthropometric traits and number of sexual partners. Taller and heavier men report more sexual partners than their shorter and lighter counterparts (Rahman et al., 2005). Caution should be 2D:4D Ratios and FA 5 taken with these findings because anthropometric differences can vary greatly among populations. However, since T exposure occurs in every culture, the 2D:4D ratio can be used to correlate T and sexual characteristics (Bogaert & Fisher, 1995). Lower ratios have also correlated with a more dominant personality, courtship personality, and conversation prowess which attracts more women (Honekopp et al., 2006). In the current study, it was hypothesized that males would have low 2D:4D ratios, high aggression, high spatial ability, and more sexual partners than females due to the organizational effects of T during development. Methods