Download 2D:4D

Correlates of 2D:4D Ratios and Fluctuating Asymmetry
Patrick J. Cooper
The Pennsylvania State University, Altoona
INTRODUCTION
The 2D:4D ratio refers to the length of one’s index finger (2D) with respect to the
length of their ring (4D) finger. Fluctuating asymmetry (FA) refers to deviation
from perfect bilateral symmetry caused by environmental stresses,
developmental instability, and genetic problems during development. It is
commonly theorized that both high FA (high variability in the measures of paired
traits) and lower 2D:4D ratios (greater length differences between the second
and fourth digits) are a result of prenatal testosterone (T) exposure. Abnormal T
exposure leads to developmental instability which results in certain personality
characteristics such as greater spatial ability, decreased verbal ability, increased
aggression, increased number of sexual partners, and early age of first sexual
experience. Past research is conclusive in suggesting that men have lower
2D:4D ratios and higher FA than women. Since men are naturally exposed to
more T, it has been suggested that prenatal T exposure results in high FA and
low 2D:4D ratios.
AIM
To see if certain personality characteristics and mental abilities are correlated
with FA and 2D:4D ratios.
METHOD
Participants 143 subjects were selected from a local university. Students signed
up for the study and received extra credit in their respective course for
participating. Males (N = 62) mean age was 23.10 years. Females (N =81)
mean age was 22.86.
Procedures Subjects completed a demographic questionnaire asking questions
about their sexual history. Then they completed the Buss and Perry Aggression
Questionnaire which measures physical and verbal aggression as well as anger
and hostility. Following this, subjects completed the Vanderberg and Kuse Mental
Rotations Test (MRT), which measured spatial ability, and the Thurstone Verbal
Fluency Test. After the tests, the length of each ear was measured to the nearest
millimeter (mm) from top to bottom using digital calipers. Each ankle’s
circumference was measured using a standard tape measure exactly 6 inches
above the bottom of the foot. Fingerprints of each of the participant’s fifth digit
and first digit were also taken. Finally, subjects’ hands were photocopied so the
digit lengths and ratios could be measured.
However, for females, these same relationships between Spatial Ability and Physical
Aggression (r = -.283, p < .014) and Total Aggression (r = -.273, p < .014) were negative.
FA was negatively correlated with spatial ability for men (r = -.266, p < .046), and
positively correlated with Verbal Aggression for women (r = .234, p < .042). The results
also indicate that there were no sex differences for number of sexual partners, verbal
fluency ability, or composite FA. Relevant means and correlations can be found in Tables
1 and 2.
Table 1
Summary of Relevant Mean Sex Comparisons
_______________________________________________________________________
ANOVAs
Mean
p-value
2D/4D ratio
.023 *
Male
.959
Female
.973
_______________________________________________________________________
Verbal Ability
.959 ns
Male
81.70
Female
81.52
_______________________________________________________________________
Spatial Ability
.0001 *
Male
22.69
Female
15.51
_______________________________________________________________________
Number of Sexual Partners
.688 ns
Male
4.29
Female
3.93
_______________________________________________________________________
FA
.589 ns
Male
14.24
Female
13.55
_______________________________________________________________________
Total Aggression
.0001 *
Male
54.32
Female
43.64
_______________________________________________________________________
Note. * = statistically significant
MEASURES
Digit Length Ratios (2D:4D) Calculated by dividing the length of the second
digit (index finger) by the length of the fourth digit (ring finger). Ratios were
calculated for both hands.
RESULTS
The results indicate that 2D:4D ratios are sexually dimorphic for the right side.
Males (M = .959, SD = .032) have significantly lower right side 2D:4D ratios
(R2D:4D) than females (M = .973, SD = .040, p < .023). This can be accounted
for by the organizational effects of Testosterone impacting the right side of the
body more than the left. Irrespective of gender, R2D:4D is significantly negatively
correlated with Physical Aggression (r = -.246, p < .003). The number of sexual
partners is also correlated with Physical Aggression (r = .198, p < .023) as well
as Hostility (r = .210, p < .015) and Total Aggression (r = .203, p <.019).
Concerning males, Spatial Ability is correlated with Physical Aggression (r =
.339, p < .007) and Total Aggression (r = .283, p < .026).
Males
Females
Mean Aggression Scores
Fluctuating Asymmetry (FA) Calculated by finding the difference between the
measures of paired traits on the body. This includes differences in ankle
circumferences, ear lengths, and the first through fourth digits. Composite FA
was compiled by adding the asymmetry from each individual trait.
16
14
*
*
NS
12
Table 2
Summary of Relevant Correlations
__________________________________________________________________________
Pearson’s r
p-value
Correlations
2D:4D
r = -.246
.003 *
Physical Aggression
__________________________________________________________________________
Total Aggression
r = .203
.019 *
Number of Sexual Partners
__________________________________________________________________________
Physical Aggression (males)
r = .339
.007 *
Spatial Ability
__________________________________________________________________________
Total Aggression (males)
r = .283
.026 *
Spatial Ability
__________________________________________________________________________
Physical Aggression (females)
r = -.273
.014 *
Spatial Ability
__________________________________________________________________________
Total Aggression (females)
r = -.240
.031 *
Spatial Ability
__________________________________________________________________________
FA (males)
r = -.266
.046
Spatial Ability
__________________________________________________________________________
FA (females)
r = .234
.042
Verbal Aggression
__________________________________________________________________________
Note. * = statistically significant
DISCUSSION
The current study reiterates the evidence that 2D:4D ratios are sexually dimorphic
traits. Males have lower ratios. This suggests that prenatal T exposure, which is more
prominent in males, is associated with lower 2D:4D ratios. Also consistent with the
literature, was the finding that males displayed more aggression and scored higher on
tests of spatial ability. It is tempting to speculate that this is mediated by T exposure.
Indeed, Total Aggression was positively associated with the number of sexual partners,
and for males, Physical and Total Aggression were positively associated with spatial
abilities – the more aggression reported, the higher the spatial scores. Interestingly,
this relationship was reversed for the female participants. That is, Physical and Total
Aggression were negatively associated with spatial abilities – the more aggression
reported, the lower the spatial scores. This finding is paradoxical in light of the fact that
low 2D:4D ratios are associated with aggression, even when examining only the
female participants. More research is needed to understand the causal pathway
between early testosterone, aggression, and spatial abilities, especially in female
participants. However, the current results are consistent with the literature in
suggesting that more male like 2D:4D ratios (early T exposure) are associated with
aggression. Finally, high levels of FA were associated with poor spatial skills in men
and more verbal aggression in women, further evidence for negative outcomes
associated with deviations from perfect symmetry.
*
10
8
PA
VA
ANG
HOS
Type of Aggression
Figure 1. Mean aggression scores for male and female participants. PA = Physical
Aggression, VA = Verbal Aggression, ANG = Anger, HOS = Hostility. * = p < .01, NS =
p > .05.
Figure 2. Hypothetical Relationship between 2D:4D ratios, Aggression, and Spatial
Scores Among the Sexes
Correlates of 2D:4D Ratios and Fluctuating Asymmetry
Introduction
-2D:4D ratio refers to the length of one’s index finger in respect to their ring finger.
-Found by dividing the length of 2D by length of 4D
-FA refers to deviation from perfect bilateral symmetry.
-Found by finding the difference between certain paired traits.
Results
-2D:4D sexual dimorphism is only significant for the right side of the body; males
have significantly lower R2D:4D ratios.
-Reason: organization effects of T dramatically influence the right side more than
the left.
-Prenatal T Exposure - Low 2D:4D ratios and high FA are a result of excessive
prenatal T exposure.
-Leads to developmental instability
Both Sexes
-R2D:4D is negatively correlated with Physical Aggression.
-Number of Sexual Partners is positively correlated with Physical Aggression.
-Number of Sexual Partners is positively correlated with Hostility.
-Sexual Dimorphism - 2D:4D is a sexually dimorphic trait with males having lower
ratios.
-Aim – To find correlates of 2D:4D ratios and FA that either support or disprove
past research
Previous Correlates of 2D:4D
-Traits previously correlated with low 2D:4D ratios include:
-Increased Aggression
-Greater Spatial Ability
-Decreased Verbal Ability
-Increased Number of Sexual Partners
Methods
-Buss and Perry Aggression Questionnaire – measures aggression
-Vanderberg and Kuse Mental Rotations Test – measures spatial ability
-Thurstone Verbal Fluency Test – measures verbal ability
Males
-Spatial Ability is positively correlated with Physical Aggression.
-Spatial Ability is positively correlated with Total Aggression.
-FA is negatively correlated with Spatial Ability.
Females
-Spatial Ability is negatively correlated with Physical Aggression.
-Spatial Ability is negatively correlated with Total Aggression.
-FA is positively correlated with Verbal Aggression.
Other Relevant Facts
-No sex differences for number of sexual partners, verbal fluency ability, or
composite FA.
Discussion
Evidence that Supports the Literature
-Males have lower ratios.
-Males are more aggressive.
-Males have better spatial ability.
-Males with higher FA have poor spatial ability
-Females with higher FA have increased verbal aggression.
-Demographics Questionnaire – sexual history
-Measures of FA include ankle circumferences, ear lengths, and digit lengths
Statistical Methods
-One way between subjects ANOVA measured sex differences.
-Pearson’s correlations measured specific correlations between variables.
New Evidence
-Low 2D:4D leads to high aggression in both males in females.
-Low 2D:4D leads to high spatial ability in males but low spatial ability in females.
-It is thought that low 2D:4D would lead to higher spatial ability and aggression
in both sexes.
-Current research suggests that low 2D:4D leads to high aggression, but
completely opposite results in spatial ability
-More research is needed to understand the causal pathway between T,
aggression, and spatial skills for females.