Introduction
Increased prenatal testosterone concentration has been shown to be inversely related to finger length, through its association with the Homeobox (Hox) genes. The hox genes are responsible for controlling the development of musculoskeletal, neurological, and gonadal tissue and thus physiological traits that may be beneficial to athletic competition (8). Expression of the Hox genes can be measured indirectly through the ratio lengths for the second and fourth digits (2D:4D ratio) (13). It is suggested that a low 2D:4D ratio (i.e., the fourth digit is longer than the second digit) is associated not only with greater success in a range of sports, many of which demand high levels of power, endurance, and visual-spatial skills (11), but also with masculine behaviors such as sensation seeking and dominance behavior (17). However, there exist a limited number of studies exploring this relationship and its value in determining the potential athleticism of an individual.
Hox gene expression controls both the overall blueprint for growth patterns of the appendicular skeleton (including the fingers and toes) (3,7,9) and the development of testes or ovaries, which are ultimately responsible for sex differences. Men and women receive small quantities of testosterone from the mother's environment in utero, and sex differences, specifically masculinized musculoskeletal, and neurological development, are attributed to higher levels of testosterone in men (8,11). This pattern of individual finger lengths is fixed by week 14 of development, so the 2D:4D ratio may be used as a proxy measure of influence of prenatal testosterone (2,8,11,14,22).
Increased prenatal testosterone may be an essential precursor for excellence in some sport activities because characteristics affected by testosterone, such as muscle fiber hypertrophy, increased strength, decreased fat mass, and an increase in hematocrit, may also contribute to the success of an athlete (1,6,11,13,16,20-23). There is also evidence that this prenatal testosterone influences cerebral hemisphere development, which has been linked to characteristics beneficial in some sports (11,25). Increased prenatal testosterone appears related to sensation seeking and aggressive behaviors while also facilitating brain development for visual-spatial abilities. A model by Geschwind and Galaburda (4) suggested that prenatal testosterone tends to slow the growth of certain areas of the left hemisphere while enhancing growth of comparable areas in the right hemisphere (4,8,11,13-16,22). Because men generally have higher levels of testosterone, brain growth would favor right hemisphere skills (11,19). This pattern of promoting neurologic growth may be advantageous for a masculinized brain and affect predisposition to athleticism (2), especially in highly dynamic sports that place an emphasis on speed, strength, visual-spatial awareness, aggression, and risk taking (11,12,15,24).
Manning and Taylor (15) assessed the 2D:4D ratios of student who participated in several sports (running, soccer, martial arts, rugby, tennis or squash, swimming, hockey) and concluded that lower 2D:4D ratios were associated with greater achievement and higher athletic attainment. This research has also assessed the 2D:4D ratio in professional sports (soccer) leagues and reported similar findings. Therefore, assessing 2D:4D ratios as an indirect measure of prenatal testosterone concentration may help predict athletic performance. The purpose of our research was to examine the 2D:4D ratio in a male population to explore relationships between division I intercollegiate athletes and nonathletes. Athletes were compared by sport, and to a control sample from the general collegiate population.
Methods
Experimental Approach to the Problem
A posttest only design was used. The independent variable was group (crew, football, gymnastics, soccer, nonathlete). These athletes were included because of the power and endurance demands of the sports. The dependent variable was the right-hand finger length ratio of the second and fourth digits (measured from photocopies) of the participants because the Hox gene relationship is stronger in the right hand than in the left (10).
Subjects
The participant sample included 138 male volunteers with intercollegiate NCAA division I athletes (92 [20.1 ± 1.4 years]) and a nonathlete group who were not varsity athletes (46 [21.1 ± 1.7 years]). Table 1 describes the sample. Participants were recruited by word of mouth and through posted advertisements. The Institutional Review Board approved the study and informed consent was obtained from each subject before participation.
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Table 1: Population by group.
Procedures
A health history and injury questionnaire was completed. Deformity to either the second or fourth digit excluded volunteers from participation. Photocopies were made (HP2210 Flatbed All-In-One, Hewlett-Packard, Palo Alto, CA, USA) of the ventral surface of the right hand of all participants (Figure 1). Finger measurements can also be taken from X-rays, but previous research has demonstrated mean 2D:4D ratios calculated from X-rays to be lower than those from photocopies (16). Digit length was measured from the tip of the finger to the crease at its base, proximal to the palm using Vernier Calipers (Model 505-633-50, Mitutoy Measurement Technology, United Kingdom) (Figure 2) (15).
Figure 1: Photocopy of hands for second digit and fourth digit measurement.
Figure 2: Vernier Caliper.
Statistical Analyses
A 1-way analysis of variance model was used to compare between groups with Bonferroni post hoc when indicated. The level of significance was set at p ≤ 0.05 for all analyses. Analyses were performed with SPSS 12.0 statistical software (SPSS Inc, Chicago, IL, USA). Measurements were made using a Vernier Caliper, measuring to 0.01 mm. All finger length measurements were made twice from photocopies. To determine intratester reliability of the procedures, the second and fourth digits on both hands of 10 subjects were measured. The same 10 subjects were measured a second time 1 day later. The finger length intraclass correlation coefficient (ICC 3,1) indicated good reliability ([ICC 3,1] = 0.967-0.996) for the intratester trials (Table 2) (18).
Table 2: Intraclass correlation coefficient for finger length measurements.*â€
Results
Significant differences were found among the different groups (F(4,137) = 5.530, p = 0.000). Significantly lower ratios were identified between football and crew (p = 0.000), football and nonathletes (p = 0.030), and gymnastics and crew (p = 0.001). Means and SDs of each group are presented in Table 3. Table 4 shows ranking of lowest ratio to highest ratio.
Table 3: Right-hand 2D:4D ratio means (athlete and nonathlete).
Table 4: Right-hand 2D:4D ratio rankings.
Discussion
The 2D:4D ratio has been suggested to provide a marker for attainment in sports (15). Although correlational research has examined the ratio and competition level within one sport, little research has compared different sports or compared athletes and nonathletes. Our research found some athletes were statistically different from nonathletes and that sport differences also exist. This provides a stronger level of evidence for examining 2D:4D ratios in the quest to predict athletic potential.
Previous research by Manning has found that skiers have lower ratios 2D:4D ratios than nonskiers (12), and that within a sample of professional soccer players, 90% had a 2D:4D ratio of less than 0.98, compared to only 50% in the control group. It was concluded the ratio of 0.98 could be considered the male-type ratio (11). Our study investigating 2D:4D ratios in division I university athletes vs. nonathletes yielded conflicting results. Of the male athletes tested, 74% had 2D:4D ratios of less than 0.98, whereas 64% of the nonathletes had ratios less than 0.98. The percentage of athletes with ratios of less than 0.98 increased when specifically examining the football players (80%) and gymnasts (88%). If the 2D:4D ratio is accepted as a proxy measure for prenatal testosterone levels, then lower ratios (high testosterone) do not automatically serve as a precursor to athletic ability, and overall contribution to a measure of athletic attainment is not evident from these data. A ratio of 0.98 may not have external validity as our data suggest nonathletes may fall under this threshold.
Neurobiological changes, such as growth of the right hemisphere, are a response to high testosterone levels, especially prenatally (15). The group differences we found suggest that football and gymnastic athletes may capitalize to a greater extent on traits that higher prenatal testosterone level would favor, whereas crew may rely less on those characteristics. One could be tempted to speculate that some men migrate toward gymnastics or football because the anatomical and physiological by-products of higher testosterone favors risk taking and aggressive sports (5,15,22). The gymnasts specifically may benefit from greater visual-spatial acuity, also associated with higher testosterone levels. However, the highest ratio, and presumable lowest prenatal testosterone level, of the crew team cannot be interpreted in a negative sense, because participation at the division I level denotes high athletic achievement and performance, regardless of testosterone levels. This observation may suggest that the generalizability of 2D:4D ratios to athletic achievement is limited to certain sports.
There are several confounding variables to consider when comparing these data to previous studies. Manning (11) described his study of English league football (soccer) players representing their country and found that although there was a low proportion of African-American participants (less than 1% of the sample), the difference in mean 2D:4D ratio between African-American and Caucasian players was significant (p = 0.001). Neither the crew nor soccer teams in our study had African-American players. Only 1 gymnast was African-American, yet 50% (15 of 30) of the football players were African-American, which may describe the differences between the sports. Only 24% (11 of 46) of the nonathlete group was African-American.
In our study, the nonathlete group was not participating in division I athletics; however, many of them had participated in sports through high school. It would have been advantageous to quantify this previous participation, or include questions regarding music, or art, where individuals may excel because of higher testosterone induced lateralization for predominantly right hemisphere skills (22). The predictive association between 2D:4D ratios and athletic achievement should be considered with caution, and if used in conjunction with other measures, may provide additional information valuable to determining athletic outcomes of individuals.
Practical Applications
The 2D:4D ratio describes the association between finger length and prenatal testosterone levels. Contrary to prior research, 2D:4D ratio of 0.98 and the ability to predict competitive level in sports was not always observed and should be considered on a sport specific basis. Individuals not competing at a higher level (i.e., university varsity athletes) may still have a low ratio. Careful analysis of the musculoskeletal, neurological, and functional performance characteristics should be considered before predictive value is asserted from 2D:4D ratios to overall athleticism; however, there are indications that information may be gleaned regarding achievement in specific sports.
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