Digit Ratio and Intelligence

So, I have been struggling to think of original things to post. Partly because my semester is coming to a close and partly because all I do is test subjects day in and day out, for reasons still cloudy to me. So I thought I would post what I have been working on: my undergrad thesis if you will. It definitely needs work (19 subjects is quite the joke.) However, it is still in the making and this beauty scored me second place here at our S&T undergrad research conference. Feel free to read up and comment, just be productive. I could use the help :)

Second to fourth digit ratio: A measure related to Intelligence

The ratio of index finger (2D) to ring finger length (4D) is an index of prenatal exposure to testosterone and estrogen (Lutchmaya, Baron-Cohen, Raggatt, Knickmeyer, & Manning, 2004). Direct measurements of these prenatal androgen levels are unavailable to researchers who wish to determine the affects of those androgens on adult behavior. Instead, researchers are looking to an alternate tactic to correlate behaviors with a accepted measure of prenatal androgen action, such as the 2D:4D ratio. Research suggests that the ratio of second-to-fourth digit length may be sexually differentiated (Kuepper & Hennig, 2007) with males, on average, having a lower 2D:4D ratio than females. This translates into males having a shorter second digit relative to the fourth digit (Manning, 2002). Prenatal testosterone masculinises the brain, resulting in higher spatial ability (Csathao et al., 2003), higher musicality (Sluming & Manning, 2000), depression and autism in men (Manning, Baron-Cohen, Wheelwright, & Sanders, 2001), and sexual orientation (Csatho et al., 2003; Lippa, Manning & Robinson, 2003).  Puts, Gaulin, Sporter, and McBurney (2004) criticizes the use of 2D:4D as an index of prenatal influence of testosterone. Puts noted that most positive results were from one lab, the exception being studies on sexual orientation, and the results can vary from equivalent to incongruous, prone to illegitimate results, the samples are exclusively one sex or the other, and most often undergraduates. Ultimately, Puts proposed that 2D:4D may be an index of prenatal testosterone exposure, but that exposure itself has different effects on the two sexes, traits, and individuals.

The fluctuating nature of these androgens during development also contributes to the dimorphic traits differentiating at indistinctive times. Broad dispositions like Woodcock-Johnson’s Cognitive Abilities or the Big Five personality dimensions, of which the development is most likely not limited to short periods of time during pregnancy, might show reliable relations with 2D:4D. With regard to intelligence, one would expect that high prenatal testosterone (low 2D:4D) causes a masculine pattern: high numerical skills, but lower verbal skills. Luxen & Buunk (2004) performed a study utilizing the relationship between verbal and numerical intelligence with 2D:4D. They found right-handed 2D:4D, Verbal and Numerical intelligence, and Agreeableness is displayed in a typical masculine pattern: low 2D:4D, low verbal intelligence, high numerical intelligence, and low agreeableness. Digit ratio might more capable of shedding light on which cognitive abilities are more masculine or feminine. However, no previous research has examined digit ratio and Woodcock Johnson III Test of Cognitive Abilities. The WCJ-III is a valuable instrument for examining intelligence because it is based on the Cattell-Horn-Carroll (CHC) theory of cognitive abilities, the only instrument that can boast this feature. The WCJ-III looks at intellectual ability, but also seven CHC factors including comprehension-knowledge, long-term retrieval, visual-spatial thinking, auditory processing, fluid reasoning, processing speed, and short-term memory. Clusters of these seven tests are utilized for test interpretation, which minimizes the danger of generalizing from the score for a single, narrow ability to a broad, multifaceted ability. This ultimately results in higher validity. One could expect high digit ratio to correlate positively with long-term retrieval, a typical female pattern. The prediction I tested for the relationship between digit ratio and the WCJ-III are as follows.

Predictions

H1: 2D:4D ratio will be positively correlated with long-term retrieval.

Method

Participants

Participants were recruited through a sign up sheet on the Research Sign up Bulletin Board in the Humanities and Social Sciences’ building. The students were offered class credit in return for their participation. 19 subjects have been recruited at the completion of this report (13 male, 6 female). Average age is 21.2 years (SD = 5.8); all 19 participants are students and 73.7% completed some college; and 94.7% of participants are Caucasian.

Materials

Ratios. The second digit and fourth digit length was measured on the subjects’ right hands in inches. The subjects placed their right hands on a clean sheet of paper. A tick mark was placed at the tip of the second and fourth digit, but careful not to mark above the nail. A second tick mark will be placed at the base of the second and fourth digit. The distance between the two tick marks was measured in millimeters. The actual digit ratio was computed by dividing the second digit length by the fourth digit length. (Mean digit ratio = 1.02, SD = .056).

Personality. The Big Five Inventory Version-44 was utilized to assess personality. The 44-item survey was chosen for the five dimensions, Openness, Conscientiousness, Extraversion, Agreeableness, and Neuroticism. A second questionnaire, intended to probe further on the anxiety scale will be administered. It is a nine-question length survey.

Cognitive Abilities. The Woodcock-Johnson was used to assess cognitive abilities. Seven tests will be administered, each assessing a different area of cognition. It will be scored, and the results will be given to the subject to keep.

Procedure

Participants were asked to complete a consent form prior to participation. The measures were counterbalanced to ensure the comprehensive batteries assessing personality and cognition, known to cause fatigue, have no lingering effects on the health measures. Each participant had a different order of tests administered. Upon completion of all the measures, they were scored and the results were provided to each participant.

Results

All analyses were performed using the statistical package SPSS 18.0. Analysis focused on the scores of each subtest of the WCJ-III and the digit ratio measurements. A significant relationship exists between 2D:4D and visual-auditory learning r(19) = .532, p = .019 (see Figure 1).  28% of variance in visual-auditory learning is accounted for by digit ratio, R² = .0283.

Discussion

I found evidence of a relation between prenatal testosterone exposure (as indexed by 2D:4D) and visual-auditory learning. Visual-auditory learning is a test of long-term storage and retrieval. On this test of associative and meaningful memory, the subject learns and recalls pictographic representations of words.  This thinking ability test requires participants to learn, store, and retrieve a series of visual-auditory associations. We found a positive correlation with visual auditory learning and digit ratio. The results make sense in the light of effects of prenatal testosterone that masculinises the brain: low testosterone exposure (high 2D:4D) makes individuals perform better with regard to long-term storage and retrieval, a typical female pattern. Low testosterone exposure results in more female typical patterns and characteristics, including the necessity of a fast, efficient, and reliable ability to store information. Evolutionarily speaking, females are delegated tasks pertaining to gathering of food for the group. This requires an ability to store and retrieve locations of the best food, the best water, etc. The efficient retrieval of this information would result in a payoff proximately in fitness, ultimately in survival.

The lack of the number of participants restrained the power of this project. In the future, more participants will be needed to determine if any other facet of the WCJ-III revealed any significant relationships with digit ratio. For example, it could be predicted that low digit ratio would exhibit a negative relationship with visual spatial abilities. I was hoping to determine if digit ratio was a stronger predictor of the variance in visual spatial than sex alone. This could be said of the seven other subtests of the WCJ-III. Studies using larger sample sizes may be able to detect these relationships between intelligence and digit ratios.