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Picture recall skills correlate with 2D:4D ratio in women but not men

Mary Poulinab, Rachael L O'Connellbc, Louise M Freemanb

1. Introduction

2. Methods

2.1. Participants

2.2. Procedure

2.3. Measurements of 2D:4D

3. Results

3.1. Sex differences

3.2. Correlations of 2D:4D ratio with cognitive measures

4. Discussion

Acknowledgment

References

Copyright

1. Introduction

The hunter–gatherer hypothesis (Eals & Silverman, 1994) suggests that evolutionary adaptation led to cognitive sex differences because the environment demanded different roles for each sex. Men were hunters who needed spatial skills, while women were foragers who needed the ability to remember the location of food (Tooby & DeVore, 1987). Successful hunting involves the same cognitive skills as the male-favored mental rotation (MR) and maze learning ability, while successful foraging requires the female-favored ability to rapidly learn the contents and spatial relationships of object arrays (Eals & Silverman, 1994).

The length of the second digit (2D) relative to the fourth (4D) is influenced by prenatal testosterone and estrogen exposure, giving men a lower average 2D:4D ratio than women, particularly on the right hand Manning, 2002, Manning et al., 1998. 2D:4D ratio correlates with a variety of sexually dimorphic behaviors, suggesting a role for developmental hormones in their expression. For example, lower 2D:4D ratios are associated with homosexuality and masculinized sex role identity in women (Csatho et al., 2003, Hall & Love, 2003, Manning, 2002, review; Williams et al., 2000). Some studies find that men with low 2D:4D ratio have higher scores on the Vandenberg–Kuse mental rotation test (VKT; Manning & Taylor, 2001, McFadden & Shubel, 2003); however, this relationship has not been replicated in other studies Austin et al., 2002, Coolican & Peters, 2003. No similar correlation of 2D:4D with MR ability has been found in women Austin et al., 2002, Coolican & Peters, 2003, Manning & Taylor, 2001, McFadden & Shubel, 2003, although blindfolded women with lower 2D:4D ratios explore mazes for longer periods of time and show better spatial memory for tactile landmarks (Csatho et al., 2001).

Here we examine the relationship between the right hand 2D:4D ratio and (1) score on a different measure of MR, the Purdue Visualization of Rotations test (ROT; Bodner & Guay, 1997), and (2) visual recall abilities, a cognitive measure in which women outperform men Eals & Silverman, 1994, Jordan et al., 2001. We expected to replicate the negative correlation between male 2D:4D ratio and VKT scores; furthermore, we predicted that one or both sexes would show a positive relationship between 2D:4D ratio and visual recall scores.

2. Methods

2.1. Participants

Participants were 218 (86 men, 132 women) undergraduate students from Mary Baldwin College, Virginia Military Institute, Washington and Lee University and Hampden–Sydney College recruited from undergraduate psychology and biology classes. The majority of participants were Caucasian. Participants with any history of injury in the second and/or fourth digits were excluded from the study. All participants gave informed consent according to the guidelines of the Institutional Review Boards at each school.

2.2. Procedure

Following completion of a demographic questionnaire, picture free recall and picture placement recall tests adapted from Jordan et al. (2001) were administered. For 20 s, participants studied a 4×4 grid with 10 PowerPoint Clipart graphics of common objects located in 10 of the squares (Fig. 1) and were asked to remember both the objects and their locations. For the free recall task, the participants were instructed to immediately turn the page and were given 2 min to write down the names of the objects they had just viewed. The participants then turned the page to a blank grid and were given 2 min to write as many names of objects as they could remember, placing the words in the location they had originally viewed the object. One point was scored for each correct response, for a maximum of 10 points per test.


View full-size image.

Fig. 1. The stimulus grid used in the picture free recall and picture placement recall tasks. Adapted from Jordan et al. (2001).


The third test was the ROT (Bodner & Guay, 1997), which measured the ability to visualize the rotation of line drawings of irregular three-dimensional objects and match a sample rotation to a different set of objects. Each question had five possible answers, only one of which matched the sample. Each participant worked two sample problems, then was given 10 min to complete the 20-item test. Each correct response earned one point.

The final tests (word free recall and placement recall) were identical to the picture free and placement recall tests except that the stimulus grid contained 10 common concrete nouns instead of pictures.

2.3. Measurements of 2D:4D

Photocopies indicating the palmar creases at the base of the second and fourth digits were made of each participant's right hand. The right hand was selected because both sex differences in 2D:4D ratio and the relationship of 2D:4D to behavioral traits tend to be stronger in the right hand than in the left (e.g., Manning, 2002, McFadden & Shubel, 2003, Williams et al., 2000). Photocopies were coded with the number of the participant's test packet.

Finger measurements from 26 participants (11 male, 15 female) were not available due to poor quality of the photocopies, but the cognitive scores for these participants were retained. 2D and 4D lengths of the remaining 192 participants (75 male, 117 female) were measured by caliper to the nearest 0.01 mm from the midpoint of the basal crease of the finger to the midpoint of the fingertip. This method of measurement is in accordance with previous studies of 2D:4D ratio (Williams et al., 2000) and correlates well with direct measurements of fingers (Manning & Taylor 2001). Two independent experimenters blind as to the cognitive test scores and the sex and appearance of the person measured the two digits. 2D:4D ratio was calculated by dividing the length of the second digit by that of the fourth digit.

The intraclass correlation coefficient showed strong agreement between the 2D:4D ratios found by the two raters (r1=.95), so the ratios from each observer were averaged to obtain the final 2D:4D ratio for each participant.

3. Results

3.1. Sex differences

The sex differences in cognitive tasks and 2D:4D are summarized in Table 1. Two-tailed independent sample t tests revealed the expected sex differences in MR test scores and 2D:4D ratio. Three-way mixed-design ANOVA showed main effects of sex, stimulus, and task (P<.05) on recall ability. Females outperformed males on both recall tasks. Pictures were better recalled than words, and free recall scores were higher than placement recall scores. In addition, there was a marginally significant interaction of sex and stimulus (P=.055). Post hoc analysis by Fisher's protected t test found significant sex differences in free recall and placement recall of pictures, but not words.

Table 1.

Sex differences in 2D:4D and cognitive abilities

Males (n=86), mean±S.E.M. Females (n=132), mean±S.E.M. t P
Mental rotationa 13.8±0.44 10.8±0.35 5.4 <.0001
Picture free Recallb 7.5±0.17 8.2±0.11 3.9 <.0001
Picture placement recallb 5.4±0.22 6.2±0.21 2.6 <.010
Word free recall 6.4±0.17 6.7±0.14 1.3 .203
Word placement recall 3.7±0.22 4.0±0.17 1.1 .278
2D:4D ratioa, c 0.96±0.004 0.97±0.03 2.7 <.05
a

Student's t test showed a significant difference in the expected direction (P<.05).

b

ANOVA followed by post hoc analysis by Fisher's protected t test showed a significant difference favoring women. (P<.05).

c

Finger measurements were unavailable from 11 men (n=75) and 15 women (n=117).

3.2. Correlations of 2D:4D ratio with cognitive measures

Because more female-like 2D:4D ratios were predicted to be associated with higher recall and lower MR scores, a one-tailed test was used. Pearson correlation coefficients for each cognitive test with 2D:4D ratio within each sex are given in Table 2. Significant correlations were found between female 2D:4D ratio and both picture free recall and picture placement recall. Correlations of female 2D:4D ratio with other variables were not significant, and there were no significant relationships between male 2D:4D ratio and any cognitive measure.

Table 2.

Pearson correlations between 2D:4D ratio and cognitive abilities in men and women

Males (n=75) Females (n=117)
Mental rotation test −.080 .023
Picture free recall −.081 .205*
Picture placement recall −.088 .237*
Word free recall .099 .011
Word placement recall .006 −.018

Significant correlations were found in women and only for those cognitive tasks in which women outperformed men (see Table 1).

*

P<.05.

4. Discussion

Our reported 2D:4D ratios were virtually identical to others collected on North American samples (Coolican & Peters, 2003). This, along with the high interrater reliability (r=.95) and the presence of the predicted sex difference in 2D:4D ratio makes us confident that we detected real variations in this measure.

As predicted, women outperformed men in visual recall tasks when pictures were used as stimuli. Higher female scores on picture placement and recall suggest enhanced visual memory abilities for objects in accordance with the hunter–gatherer hypothesis (Eals & Silverman, 1994). Notably, the only visual recall tasks that correlated with 2D:4D ratio were those in which females outperformed males. The results from our study are consistent with the hypothesis that fetal hormones affect 2D:4D ratio while also directly or indirectly influencing visual recall abilities in females, with the reproductively advantageous lower androgen and higher estrogen levels producing both more feminine hands and better visual recall. To our knowledge, this is the first demonstration of a relationship between 2D:4D ratio and a cognitive ability favored by women. We detected no such relationship between 2D:4D and visual recall in men.

Like others Austin et al., 2002, Coolican & Peters, 2003, we failed to replicate the negative correlation of 2D:4D ratio with MR for men reported previously by Manning and Taylor (2001) and McFadden and Shubel (2003). Also in accordance with previous work Austin et al., 2002, Coolican & Peters, 2003, Manning, 2002, McFadden & Shubel, 2003, we found no relationship between 2D:4D ratio and MR ability in women. Taken together, male MR abilities appear mildly sensitive to early sex steroids as measured by 2D:4D ratio, but the relationship is weak and difficult to detect consistently. In women, an even weaker and so far statistically insignificant relationship with MR ability is seen. This suggests that either (1) MR skill is less sensitive to developmental sex steroids in females or (2) the sensitive period for females occurs at a different time point than that of 2D:4D ratio differentiation (McFadden & Shubel, 2003).

As with MR, the association of visual recall to 2D:4D ratio is found only in one gender; in this case, the female. This pattern is similar to the one recently reported by Austin et al. (2002), who found a relationship between sensation-seeking tendency and 2D:4D ratio in women only. This raises the question: If correlations of such behavioral traits with 2D:4D ratio are a result of early hormone influence on those behaviors, why are those effects seen in one sex rather than both? In contrast, early hormone influences on mammalian reproductive behaviors show phenotypic effects in both sexes (Breedlove, 1992). Although there are some inconsistencies in reported results, the association between sexual orientation and 2D:4D ratio has been reported in both men and women Manning, 2002, Williams et al., 2000.

The same continuum of selective pressures was not necessarily in place for nonreproductive as for reproductive behaviors. If, for instance, in accordance with the hunter–gatherer hypothesis, good MR skills were selected for in male human ancestors, it does not necessarily follow that such skills would be selected against in females. Similarly, good object placement recall may have been adaptive for women, but not necessarily maladaptive in men. If selective pressures influenced the evolution of these skills by making them in some way malleable to the same sex steroids that determine reproductive fitness, those selective pressures would have acted in one sex only. Therefore, one might expect specific cognitive skill sensitivity to developmental hormones only in the sex in which that particular skill has been specifically selected for or against.

Another study (Csatho et al., 2001) found that women with lower 2D:4D ratio tended to explore a three-dimensional maze longer during training and better recalled the type and location of landmarks afterwards. The apparent sensitivity of maze-solving ability to early hormones in women is not necessarily inconsistent with the sex-specific sensitivity model. First, women in Csatho et al.'s (2001) study were navigating through the maze using tactile cues while blindfolded. Hunter–gatherer human ancestors presumably relied heavily on visual input for both the MR and spatial learning tasks favored by men and the placement recall tasks favored by women. Nonvisual aspects of maze solving may not have been under the same selective pressures. Second, the superior landmark recall seen in women with lower, more masculinized 2D:4D ratios followed a longer latency to find the escape during training, which Csatho et al. interpreted as a measure of exploratory tendency. If women with lower 2D:4D tended to explore the maze longer during training, their improved ability to correctly place landmarks may be the result of a greater opportunity to form a detailed cognitive map, rather than an improved tendency to recall it. In fact, the latency to locate the escape on probe trials did not significantly correlate with 2D:4D ratio (Csatho et al., 2001). The improved spatial memory (measured by reduction of escape latency between training and probe trials) seen in women with more male-like 2D:4D ratios may therefore be an artifact of increased exploratory tendencies. Exploratory behavior may be related to sensation seeking, which, like picture recall abilities, correlates with 2D:4D ratio in women but not men (Austin et al., 2002).

Correlations of 2D:4D ratio with cognitive abilities suggest some role of developmental sex steroids in the development of sex differences. Our results indicate that the female-favored trait of visual recall ability may be, in part, organized by early sex steroids.

Acknowledgements

The authors would like to thank Jennifer Powell and Colleen Greenwood for assistance with data collection and analysis, S. Marc Breedlove for methodological advice, the Stewart Equipment Company of Staunton, VA for the loan of the portable photocopier and the students of Washington and Lee University 2001 Summer Scholars program in Brain and Behavioral Studies, who collected valuable pilot data for this study. This paper is based on the thesis submitted by M. Poulin in partial fulfillment of the requirements for the Bachelor of Arts degree in psychology at Mary Baldwin College.

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a Adult Degree Program, Mary Baldwin College, Staunton, VA 24401, USA

b Department of Psychology, Pearce Science Center, Mary Baldwin College, Staunton, VA 24401, USA

c Virginia Women's Institute for Leadership, Mary Baldwin College, Staunton, VA 24401, USA

Corresponding author. Tel.: +1-540-887-7326; fax: +1-540-887-7121

PII: S1090-5138(04)00020-0

doi:10.1016/j.evolhumbehav.2004.03.004



2007:11:13