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Original Research

Digit ratio (2D:4D) and performance in male surfers

Kilduff, Liam P; Cook, Christian J; Manning, John T

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Journal of Strength and Conditioning Research: November 2011 - Volume 25 - Issue 11 - p 3175-3180
doi: 10.1519/JSC.0b013e318212de8e
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Abstract

Introduction

The relative length of the second (index) and fourth (ring) fingers (digit ratio or 2D:4D) shows a sex difference in that men tend to have lower values of 2D:4D relative to women (for reviews, see [9,15]). The sexual dimorphism in 2D:4D is established in the fetus (8), and it has been suggested to be dependent on fetal testosterone and estrogen levels (i.e., prenatal testosterone is thought to be negatively related to 2D:4D and prenatal estrogen positively related to 2D:4D; 17). Sex steroids have substantial ‘organizing’ effects on fetal organs; thus, 2D:4D may be a predictor of sex-dependent traits including performance in sports (4,5,18).

The link between 2D:4D and sports performance was first explored by Manning and Taylor (18). They found a negative relationship between 2D:4D and performance in a range of sports in a sample of young men. In addition, a sample of male professional footballers playing in the English leagues showed similar trends, such that low 2D:4D was found more frequently in (a) players in the Premiership compared to in lower league players, (b) first-team players compared to reserves, (c) international players compared to noninternationals, (d) international players with many caps compared to international players with few caps (12). More recently, similar associations between 2D:4D and performance have been found in male professional rugby players, with low 2D:4D showing associations with number of international appearances and number of tries scored (1). However, the associations between 2D:4D and sport are not restricted to ball games such as football and rugby. Since the Manning and Taylor (18) study, low 2D:4D has been shown to be linked to male performance in skiing, middle- and long-distance running, sprinting, and various measures of physical fitness (for a meta-analysis of the field see [5]).

Surfing is a water sport where the surfer moves along the face of a wave while balancing on a board. Surfing begins when the surfer finds an appropriate wave and then matches its speed. When the wave begins to carry the surfer forward he or she stands up on the board and rides down the surface of the wave, staying just ahead of the white water or breaking part of the wave. It has been suggested that success in competitive surfing is dependent on the interrelationship between the surfer's tactical, cognitive, psychological, technical, biomechanical, and physiological capacity (19) making it a complex sporting action. From a practical viewpoint, 2D:4D may offer a marker for talent identification in young surfers. The relative lengths of the fingers are determined in individuals by the end of the first trimester (8) are stable across childhood, puberty, and into adulthood and are not influenced by between-individual maturational differences (22). If performance in surfing is related to low “masculinized” ratios this may mean that low 2D:4D could be used by coaches to identify young surfers of high potential.

Although 2D:4D has been shown to correlate with performance in a number of sports the strength of the link varies substantially across sports. Hönekopp and Schuster (5) have shown that correlations between 2D:4D and male sports performance average r = −0.27. However, around this mean, there is considerable heterogeneity in the strength of associations. Moreover, this heterogeneity is probably not accounted for solely by sampling effects. For example, male performance in middle- and long-distance running tends to show correlations with 2D:4D of about r = −0.50 (16), whereas sprinting speed is much less strongly related to 2D:4D with r = −0.15 (15). The reason for these differences in effect size in the links between 2D:4D and sport presumably relates to the underlying trait that is predicted by 2D:4D. At present, we do not know what these are. Associated with the issue of different effect sizes across sports is the question of laterality effects on 2D:4D. Right 2D:4D is more strongly sexually dimorphic than left 2D:4D, and this may be because right 2D:4D is a stronger correlate of prenatal testosterone (6,9,10). Both right and left 2D:4D have been found to be linked with sports performance (5), but the considerable heterogeneity in the strength of these links suggests that for some sports right 2D:4D is the best predictor (e.g., in rugby, [1]), but in others, left 2D:4D shows stronger links to performance (e.g., football, [12]). In addition, right–left 2D:4D (or Dr–l) is a third predictor variable of sports performance in some studies. The Dr–l is sexually dimorphic with men showing low Dr–l compared to women (13), and Dr–l may correlate negatively with prenatal testosterone (9). Some studies have reported Dr–l to be a stronger predictor of sports performance than right or left 2D:4D (e.g., Bennett et al. [1]). Given this variation in effect sizes and laterality effects in the link between 2D:4D and sport in men, it would be helpful to examine associations between 2D:4D and a greater range of sports. In this way, we may clarify which sports are strongly associated with 2D:4D and which sports show marked laterality effects in their associations with right or left 2D:4D or Dr–l. With this in mind, we examined associations between 2D:4D (right or left 2D:4D or Dr–l) and performance in surfing.

Methods

Experimental Approach to the Problem

A total of 46 male surfers completed this study. All surfers had their second and fourth fingers of both hands measured using vernier callipers before a Men's Professional World Qualifying Series (WQS) surfing competition. After the end of the surfing competition, 3 experienced coaches ranked the surfers to measure surfing ability (over the whole competition season). In addition, each participant's final placing in the Newquay competition was noted as an additional performance measure. Subjects height and body mass were also recorded.

Subjects

The study was approved by the local ethics committee, and the participants gave informed consent. Forty-six male surfers (means and SDs) were as follows: age 23.65 (4.72) years; height 177.11 (5.38) cm; body mass 73.07 (4.55) kg; surfing experience 13.94 (4.49) years were measured over a period of 2 days during a Men's Professional WQS surfing competition which took place in Newquay, United Kingdom, on August 5–9, 2009. The main aim of the WQS is to select those who will be permitted to enter the Association of Surfing Professionals world tour. To this end, each WQS competition is allocated a star rating (1–6 stars), and at the end of the season, surfers are allowed to tally their rating. The Men's Newquay competition has a 5-star rating.

Anthropometrics

The second and fourth fingers of the right hand were measured using vernier callipers (Mitutoyo 505-633-50) measuring to 0.01 mm. The ventral surface of the finger was measured, from a midpoint on the crease proximal to the palm to the tip of the finger (see [17] for methodology). The fingers of the right hand were measured first and then the left hand, and this was repeated twice so that there were 3 measurements per finger. The 2D:4D ratio was calculated by dividing 2D by 4D (2D/4D), and Dr–l by subtracting left 2D:4D from right 2D:4D (right 2D:4D–left 2D:4D).

It is known that 2D:4D is dependent on ethnicity, with in general high mean 2D:4D among Caucasian groups and lower mean 2D:4D among East-Asian and Black groups (9,16). The surfers were from 7 countries (Australia, Brazil, Israel, South Africa, United Kingdom, Hawaii, and mainland USA), and all were Caucasian (i.e., they were not of East-Asian or Black origin). Therefore, 2D:4D was not adjusted for ethnicity.

Performance Measures

To measure surfing ability, 3 experienced coaches, who were members of the British Surfing Association, were asked to rank order (1–46; 1 = the best) the surfers as to their overall surfing ability independent of their age and experience. This was our main index of performance. In addition, each participant's final placing in the Newquay competition was noted.

Statistical Analyses

With regard to statistical analysis, intraclass correlation coefficients were used to assess the repeatability of 2D:4D. Considering the links between 2D:4D and surfing performance, the distribution of the coaches' rankings was not parametric (i.e., they were rectangular). Therefore, nonparametric correlation analysis (Spearman Rank Correlation, rs) was used. Where it was necessary to remove the effect of other variables (such as surfer's mass) from 2D:4D, we used the residuals of 2D:4D regressed on the related variable.

Results

Repeatability of 2D:4D

Right and left 2D and 4D lengths were each measured 3 times. This allows for the calculation of 6 values of 2D:4D (i.e., 3 right 2D:4D and 3 left 2D:4D). We used repeated-measures analysis of variance to calculate intraclass correlation coefficients (r1) for right 2D:4D and left 2D:4D. These were as follows: right 2D:4D r1 = 0.946, F = 53.46, p = 0.0001; left 2D;4D r1 = 0.980, F = 150.22, p = 0.0001. We concluded that random error in the measurement of 2D:4D was small in relation to real between-individual differences in 2D:4D. Therefore, we calculated mean finger length and mean right and left 2D:4D (SD). The latter were as follows: right 2D:4D 0.944 (0.023); left 2D:4D 0.976 (0.028); right–left 2D:4D (Dr–l) −0.033 (0.026). Right and left 2D:4D were significantly positively correlated (r = 0.46, p = 0.001). The right 2D:4D was significantly less than the left 2D:4D (paired t test, x–y = −0.033, t = 8.54, p = 0.0001).

Predictors of Surfing Performance

The coaches' ratings of overall surfing ability were used as the main measure of surfing ability. A single competition placing is open to chance influences. However, we also considered the Newquay competition placing.

Coaches' Oatings of Overall Surfing Ability

The ratings for coaches 1, 2, and 3 and their mean per surfer are shown in Table 1, together with right 2D:4D, left 2D:4D, and right–left 2D:4D (Dr–l). We found the correlations between coaches' ratings to be significant (p = 0.0001) for all combinations (coaches 1 and 2 rs = 0.66; coaches 1 and 3 rs = 0.80; coaches 2 and 3 rs = 0.82). Therefore, we calculated the mean of all 3 coaches' ratings and used this as our measure of performance.

T1-32
Table 1:
Rankings of overall surfing ability from 3 coaches and the mean of these 3 rankings, together with right 2D:4D, left 2D:4D, and right–left 2D:4D (Dr–l) for 46 surfers competing in the Newquay 5-star WQS surfing competition, 2010.

The relationships between mean of all 3 coaches' rankings and right 2D:4D, left 2D:4D, Dr–l and age, height, weight, and surfing years are given in Table 2. There were 2 significant relationships, that is, a correlation of quite high coefficient of determination (r2) between right 2D:4D and ratings (rs = 0.58, r2 = 0.34, p = 0.0001) and a weaker correlation between mass and ratings (rs = 0.32, r2 = 0.10, p = 0.03). That is, surfers with low 2D:4D (high prenatal testosterone and low prenatal estrogen) and lighter surfers were rated most highly.

T2-32
Table 2:
Relationships (Spearman rank correlation coefficients, r s) between 7 potential predictor variables and mean of coaches' rankings of performance in surfing.*

We removed the effect of mass on right 2D:4D by regressing right 2D:4D on mass and considered the residuals (resright 2D:4D onmass) from this analysis. The correlation between resright 2D:4D onmass and ratings was rs = 0.51, r2 = 0.26, p = 0.0001 (Figure 1). Therefore, the relationship between right 2D:4D and surfing ability was largely unaffected by the influence of surfers' mass.

F1-32
Figure 1:
The relationship between right 2D:4D independent of weight (resright2D:4D onmass) and coaches' ratings.

Newquay Finishing Place

We found a significant correlation between the average coaches' score and the final Newquay placing (rs = 0.45, r2 = 0.20, p = 0.003). There were no significant relationships between the Newquay placing and age, height, mass, surf years, left 2D:4D or Dr–l. However, there was a significant positive correlation between right 2D:4D and placing (r = 0.30, r2 = 0.09, p < 0.05), such that competitors with low 2D:4D tended to finish higher in the finishing order than competitors with high 2D:4D (Table 3).

T3-32
Table 3:
Relationships between 7 potential predictor variables and placings at the WQS 2010 Newquay surfing competition.*†

Discussion

We have found the following in a sample of 46 surfers: (a) mean right 2D:4D was 0.944 (0.023) and left 2D:4D was 0.976 (0.028), the difference was significant (b) performance in surfing (as judged by the ratings of 3 coaches) was quite strongly negatively correlated with right 2D:4D (rs = 0.58, r2 = 0.34, p < 0.0001) but was not significantly related to left 2D:4D, Dr–l or age, height or years of surfing (c) the mass of surfers was also negatively correlated with performance in surfing but with moderate effect size (rs = 0.32, r2 = 0.09, p = 0.03 (d) after removal of the effect of mass we found right 2D:4D remained a significant negative correlate of performance in surfing (rs = 0.51, r2 = 0.26, p < 0.0001).

As in studies of professional footballers (12) and rugby players (1), we have found values of 2D:4D in surfers that are low in relation to male population norms. For example, values for surfers (0.944 for right 2D:4D and 0.976 for left 2D:4D) and rugby players (0.963 for right 2D:4D and 0.956 for left 2D:4D) are lower than population norms for 2D:4D (0.985 for right 2D:4D and 0.975 for left 2D:4D), reported by Bennett et al. (1). This may indicate that sportsmen in general have been exposed to higher prenatal testosterone (if this assumption is true) than nonsportsmen.

The relationship between right 2D:4D and surfing performance is quite strong and is of a similar strength to correlations between 2D:4D and speed in middle- and long-distance running (16), aspects of performance in soccer (first-team squad vs. reserves; 12) and in rugby (number of international caps and number of tries scored [1]). In these sports, 2D:4D explains about 20–27% of the variance in performance. Values of r2 of between 10 and 20% for links between male 2D:4D and performance have been reported for slalom skiing speeds (9) and soccer ability rated by self and others (12). Although r2 of <10% have been found for sprinting speed (15), gym-based fitness tests and PE mark in schools (4) and hand-grip strength (2). Studies that have reported correlations between 2D:4D and highest level of sport participation in samples made up of a number of sports have shown r2 values of about 10% (18,21). Overall, this pattern appears to confirm the suggestion that the link between 2D:4D and performance is stronger in those sports that emphasize cardiovascular efficiency rather than strength (13,15). In support of this suggestion, Hönekopp and Schuster (5) found that the relationship between 2D:4D and running performance increased in strength as distance increased.

With regard to laterality effects on 2D:4D, we have found strongest effects for right 2D:4D. This is often the case for 2D:4D studies. Thus, in the first “modern” study of 2D:4D, Manning et al. (17) reported significant right 2D:4D (but not left 2D:4D) correlations with male fertility parameters (ejaculate size and sperm characteristics) and steroid hormones. This preponderance of right-side significant effects has been found in other aspects of 2D:4D studies, for example, the effect size of the sex difference in 2D:4D is the greatest on the right than on the left side (16), and right 2D:4D is lower (more “masculinized”) than the left 2D:4D (7). Despite these widespread laterality effects, a meta-analysis of effect sizes for links between 2D:4D and sport concluded that there were no side differences. However, 2 subsequent studies (rugby [1]; and this study) have shown a stronger link for right 2D:4D compared to left 2D:4D.

It appears likely that 2D:4D is predictive of one trait (or a few interrelated traits) that underlies the link between 2D:4D and male sport performance. The trait is unlikely to be related to cognitive abilities or aspects of personality. Thus, Tester and Campbell (21) found a significant negative correlation between 2D:4D and performance in a sample that was made up of male basketball, rugby, and football players. The relationship was little affected by scores on a mental rotation task or by measures of assertiveness, achievement, control, and harm avoidance. In addition, a meta-analysis of studies on 2D:4D and male aggression found an overall significant but weak negative correlation (r = −0.06) that is unlikely to be a major factor in the link between 2D:4D and sport (7).

Thus, it appears likely that 2D:4D is a correlate of a physiological trait that is important in sport. Both Testosterone and estrogen (in utero) may be linked to this trait. In a recent meta-analysis, Hönekopp (3) and Watson (7) have found little evidence that 2D:4D is a correlate of adult levels of either hormone. However, there is accumulating evidence that low 2D:4D is associated with high prenatal testosterone (see [7], for meta-analysis of appropriate studies). How high fetal testosterone is linked to sport performance is not clear. However, it may be that the formation of the vascular system is affected by early testosterone levels. In support of this, it has been reported that atherosclerotic plaque development (20) and early myocardial infarction in men (11,12) are both linked to high values of 2D:4D.

In conclusion, we have found that low values of 2D:4D in the right hand are correlated with male performance in surfing such that variation in 2D:4D explains about 25% of the variance in surfing performance. In relationship with other studies of the link between 2D:4D and sport, this r2 value is quite high and is similar in magnitude to correlations between 2D:4D and middle- and long-distance running and rugby and soccer performance.

Practical Applications

We emphasize that the practical aspect of 2D:4D use in surfing relates to talent identification. The relative proportions of the finger lengths are determined very early in fetal growth (8), and cross-sectional (17) and longitudinal (22) studies have shown that they remain stable throughout childhood and puberty. Thus, a boy of say 10 years with a low (“masculinized”) 2D:4D will continue to have a low ratio in his teens and as an adult. That is, 2D:4D is unaffected by variation in maturational rates. This means that coaches' may use 2D:4D as an additional indicator in talent identification when choosing young surfers with potential to develop to a high standard.

References

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2. Fink, B, Thanzami, V, Seydel, H, and Manning, JT. Digit ratio and hand-grip strength in German and Mizos men: Cross-cultural evidence for an organizing effect of prenatal testosterone on strength. Am J Hum Biol 18: 776–782, 2006.
3. Hönekopp, J, Bartholdt, L, Beier, L, and Liebert, A. Second to fourth digit length ratio (2D:4D) and adult sex hormone levels: New data and a meta-analytic review. Psychoneuroendocrinology 32: 313–321, 2007.
    4. Hönekopp, J, Manning, JT, and Muller, C. Digit ratio (2D:4D) and physical fitness in males and females: Evidence for effects of prenatal androgens on sexually selected traits. Horm Behav 49: 545–549, 2006.
    5. Hönekopp, J and Schuster, M. A meta-analysis on 2D:4D and athletic prowess: Substantial relationships but neither hand out-predicts the other. Pers Individ Diff 1: 4–10, 2010.
    6. Hönekopp, J and Watson, S. Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand. Am J Hum Biol 22:619–630, 2010.
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    11. Manning, JT and Bundred, PE. The ratio of second to fourth digit length and age at first myocardial infarction in men: A link with testosterone? Br J Cardiol 8: 720–723, 2001.
    12. Manning, JT, Bundred, PE, and Taylor, R. The ratio of 2nd and 4th digit length: A prenatal correlate of ability in sport. In: Kinanthropometry (Vol. VIII). Reilly, T and Marfell-Jones, M, eds. London: Routledge, Taylor and Francis Group, 2003. pp. 165–174.
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    14. Manning, JT, Fink, B, Neave, N, and Caswell, N. Photocopies yield lower digit ratios (2D:4D) than direct finger measurements. Arch Sex Behav 34: 329–333, 2005.
    15. Manning, JT and Hill, MR. Digit ratio (2D:4D) and sprinting speed in boys. Am J Hum Biol 21: 210–213, 2008.
    16. Manning, JT, Morris, L, and Caswell, N. Endurance running and digit ratio (2D:4D): Implications for fetal testosterone effects on running speed and vascular health. Am J Hum Biol 19: 416–421, 2007.
    17. Manning JT, Scutt, D, Wilson, J, and Lewis-Jones, DI. The ratio of 2nd to 4th digit length: A predictor of sperm numbers and levels of testosterone, LH and oestrogen. Hum Reprod 13: 3000–3004, 1998.
    18. Manning, JT and Taylor, RP. Second to fourth digit ratio and male ability in sport: Implications for sexual selection in humans. Evol Hum Behav 22: 61–69, 2001.
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    22. Trivers, R, Manning, JT, and Jacobson, A. A longitudinal study of digit ratio (2D:4D) and other finger ratios in Jamaican children. Horm Behav 49: 150–156, 2005.
    Keywords:

    surfing; prenatel testosterone; second and fourth digits

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