PerceptuaZand~UotovSkzlls,2008, 107,3-13. O Perceptual and Motor Skills 2008
RATIO O F FOURTH T O SECOND FINGERTIP EXTENSIONS IN RELATION T O SERUM ESTRADIOL AND TESTOSTERONE LEVELS IN ApIEN AND \7CTOhIEN1
UNEK TAN
Sz~7rzmal-;.-Sex differences in second (2D) and fourth (4D) fingertip extensions relative to the iniddle fingertip and 4D:2D fingertip extension ratios were studied in men and women. Body height positively corrclatcd with index fingertip extensions, not with ring fingertip extensions; nor with their ratio. Mean 2D extension (both hands) was smaller in women than men: mean 4D extension (right hand) was smaller in mcn than women: 4D:2D fingertip extension ratios from both hands were larger in women than men. Serum estradiol concentration negatively correlated with 2D extensions for both hands (no significant correlation with 4D extensions), but positively correlated with 4D:2D extensioll ratios for both hands. Serum testosterone concentration positively correlated with 2D extensions of both hands (no significant correlation with 3D extension); but negatively correlated with 4D:2I) extension ratios for both hands. These relations were also studied in men and women separately. It was concluded that the 4D:2D extension ratio was greattr in women than in men; 2D and 4D extensions and 4D:2D extension ratios nyay be determined prenatally by sex hormones; finpertip extensions may be predictive of adult and prenatal sex hormone le\:els.
Research on linger-length patterns, i.e., ratio of the index to the ring finger length (2D:4D), essentially started with a leading article from Manning, Scutt, Wilson, and Lewis-Jones (1998) and is acculnulating nearly in every aspect of life sciences. There are reports suggesting associations of 2D:4D ratios to sex, prenatal sex hormone lelels, ethnicity, psychological traits (cognition and personality), fertility, heart disease, obesity, psychological disorders (autism, depression, schizophrenia), sports, physical abilities, sexual orientation (heterosexual men and women, lesbians, gays, transsexuals), immunitl, human development, and growth (see hIannlng, 2002). Before being attributed a universal indicator, the finger-length patterns were reported much earlier by Baker (1888), George (1930), hlinami (1952), and Legay and Heizmann (1967). However, the historically earliest reports (e.g., Ecker, 1875: Baker, 1888) were on the distal extents of fingertips relatire to the middle fingertip, contrary to the now commonly used Manning method of measuring the finger lengths from the most proximal to the most distal ends of the fingers. Peters, Tan, Kang, Teixeira, and Mandal (2002) used the original meth-Address correspondence to Prof. Dr. Uner Tan, Cukurova University, Faculty of Arts and Sciences, 01330 Adana, Turkey or e-mail (
[email protected]).
od, measuring the fingertip extensions, to study the sex-specific fingerlength patterns in various human populations. The results seemed to be consistent with expected sexual dimorphism in the 2D:4D digit ratio reported by many authors studying finger-length patterns (e.g., hIanning, et nl., 1998; Tester & Campbell, 2007: Voracek, Reimer, Ertl, & Dressler, 2006; Voracek & Dressler. 2007). The diflerence in the lengths of the distal extents between index and ring fingers, relative to the middle fingertip, is smaller in females than in males, suggesting nearl) equal-length index and ring fingers in females as compared to males with longer ring than index fingers. The importance of the theory of sexually dimorphic 2D:4D digit ratio is the supposed prenatal origin of this trait: high prenatal testosterone and lo^ estrogen (lour 2D:4D ratio) may be responsible for the male pattern, while low prenatal testosterone and high estrogen may be responsible for the female pattern. Accordingly, Manning. et al. (1998) have shoxrn in adults that the 2D:4D ratio is negatively correlated with serum testosterone concentrations and positively correlated with serum estrogen concentrations, suggesting that men with lour 2D:4D ratios would ha\e had more testosterone and less estrogen prenatally than men with high ratios. These authors did not clarify if they measured the free or total testosterone concentrations. On the other hand, the 2D:3D ratio was positively correlated with serum estrogen levels in \\-omen, suggesting that women with high digit ratios u~ouldhave had more estrogen prenatal13 than women with low digit ratios. It was argued that the above results in adults would reflect prenatal hormone levels, since digit ratios are determined prenatally by about the 13th week (Phelps, 1952; Garn, Burdi, Babler, & Stinson. 1975). If adult sex hormone levels are correlated with phjsical traits such as digit ratios, then another sex hormone-dependent trait, body height, should also be related to digit ratios in adults. However, there are inconsistent reports on this rarely studied subject. For instance. Manning, et nl. (1998) could not find any significant relationship between height and digit ratios, similar to Fink, Neave, and Manning (2003) and Neave, Laing, Fink, and Manning (2003). However, Tester and Campbell (2007) reported a significant negatir-e correlation between height and digit ratio ( r = -.25, p < .05). Distal fingertip extensions relative to the middle finger were measured in the present study instead of the whole finger lengths, since there is a tiny sex difference in the digit length ratio (usually 0.98 mm in males and 0.99 mm in females), about .25 standard deviations (see Honekopp. Bartholdt, Beier, & Liebert, 2007). Such a small difference uould be vulnerable to many factors and easily disappear from sample to sample depending upon too many factors. For instance, there was no significant sex difference in 2D:dD digit ratios in 300 English adults (Manning, Barley, KTalton, LewisJones, Trivers, Singh, et nl., 2000). Similarly, there are indeed a number of
FINGERTIP EXTEKSIONS AND HORTTIONE LEVELS
5
other null findings in the scientific literature (see Putz, Gaulin, Sporter, & hlcBurnley, 2004). Therefore, replication is desirable. Null results are generally not published, for example, an earlier attempt to study the sexual dimorphism in 2D:4D ratios of some Turkish samples (Tan & Okuyan, unpublished observations). In contrast, Peters, et al. (2002), measuring distal fingertip-extension patterns, found larger sex differences than had been observed for digit ratios. For these reasons, the method of measuring the finger extensions relative to middle fingertip was used in the present study. Body height, another hormonally influenced and sexually dinlorphic physical trait, as well as serum estradiol and free testosterone concentrations, were studied in relation to fingertip extensions, to study the proposed hormonal correlates of sexuallj7 dimorphic finger-length patterns considering the relation of adult sex hormone concentrations to prenatal sex hormone levels (see Jamison, hleier, & Campbell, 1993). Folloxving these considerations, it was expected that: ( I ) a greater sex difference urould be found using measurements of fingertip extensions relathe to middle fingertip than has been previously reported; ( 2 ) body height would be a significant predictor of fingertip extensions; (3) lengths of fingertip extensions and 4D:2D extension ratios would be significant predictors of adult sex hormone concentrations. ~~ETHOD
The participants (264 women, 214 men) were Turkish university students urho voluntarily participated in the study. Ages ranged from 20 to 22 bears. The extensions of the index i2nd) and ring (4th) fingertips relative to the middle fingertip were measured using the apparatus originally built by George (1930) and rebuilt in Pvlichael Peters' laboratories (see Peters, et al., 2002). The participants were asked to place their palms flat on a surface and to bring the tip of the middle finger into gentle contact ~ i t ah fixed vertical block. There were parallel sliders on the right and left sides. The assistant moved these sliders until their vertical surfaces came into gentle contact with the tips of the index and ring fingers, so that the extensions of the 2nd and 4th fingertips relative to the middle fingertip could be measured in 0.5-mm steps. Using this method, the distances (extensions) between the tip of the middle finger and the tips of the index and ring fingers can be accurately measured. The reliabilities of measurements from the same investigator wcre assessed using the intraclass correlation coefficient routine (ICCs, two-way mixed effects model with absolute agreement definition) in SPSS Version 12. The ICCs (X=21) were found to be .94 and .99 for the 4D:2D extension ratios from the right and left hands, respectively (right hand, F,,= 27.07, p < .001; left hand, F,, = 129.3, p < .0001). Sex differences in fingertip extension ratios were also assessed with effect size (q2),Body height was recorded in cm with 1-mm accuracy.
The fingertip extension ratio 4D:2D was analyzed in the present study instead of the ratio 2D:ID in analysis of finger-length patterns, to make the results more consistent with studies related to finger-length patterns. Peters, Mackenzie, and Bryden (2002) have also used the fingertip extension ratio 4D:2D in their study related to fingertip measurements. To assess the serum testosterone and estrogen concentrations, intravenous blood samples were taken between 10:OO and 14:00, when the fingertip extensions were measured. After collecting the blood sample, the blood mias stored at -IO°C. Serum concentrations of free testosterone and estradiol were measured by radioimmunoassay using commercial kits (Diagnostic Systems Laboratories, Webster, TX, USA).
Index Finger ( 2 0 ) Sex diffeere?zces.-The mean index fingertip extensions relative to the middle fingertips of the right and left hands were significantly greater in males than in females (see Table 1). Sex had a significant main effect on the dependent variables, right and left index fingertip extensions (F,,4,1 = 60.3 1, p < .OOOI; q2= 20). The sex difference was more pronounced for the right hand than the left hand, namely, the mean difference between the index fingertip extensions of males and females was found to be 2.7 mm (SE=0.2) for the right hand, and 1.5 mm (SE=O.3) for the left hand. The mean index fingertip extension for the right hand was significantly greater in males than TABLE I EXTENSIONS (MM) OF SECOND AYD FOURTH FINGERTIPS RELATIVE TO MIDDLEFIUGFRTIPS I N 'CIA1F 5 ( t = ~214) AND F E M ~ L (n E S= 264) -
-
F~npertloExtensron
Left Hand 2D
Sex
11.1
Female Male Female Male Female Male
9.1 11.8 9.2 8.9 1.1 0.8
SD
F
D
rlz
Female 10.i Male 12.1 Female 8.7 Male 9.3 Fernale 0.9 Male 0.8 -... A\iotr.-2D = index fingertip extension; 4D = ring fingertip extension; 4D:ZD = ratio of ring to index fingertip extensions.
FINGERTIP EXTENSIONS AND HORMONE LEVELS
7
in females (F,,,,= 118.69, p < .OOOI; q 2 =.20), similar to the left hand for which there was a smaller effect size (F,,,,=31.44, p < .0001; q 2 =.06). Since there was a significant positive correlation between the index fingertip extensions and the body height (see Table Z), the ratios of the lingertip extensons to body heights were used instead of the raw measurements. Using this procedure yielded essentially similar results, showing significantly less extended index fingertips in females than in males (F,,,,~ 5 . 2 8 p. < .01; q2= .08). However, the mean differences between males' and females' fingertip extensions were almost equal for the right and left hands, 0.009 mm (SE= 0.003) for the right hand and 0.009 m n ~(SE=O.O03) for the left hand. So the inore pronounced sex difference in the right hand was due to the effect of body height. Estradzol and testosterone.-Serutn estradiol concentration showed a negative correlation with the index fingertip extensions of the right and left hands in women (see Table 3). This suggests that high serum estradiol levels may be related to smaller index fingertip extensions from the right and left hands of females (see Table I). On the other hand, serum estradiol concentrations had a positive correlat~onwith index fingertip extensions in men (see Table 3). In comparison to estradiol, serum testosterone levels were found to be positively correlated with the index fingertip extensions of the right and left hands of both women and men (see Table 3). Higher testosterone concentrations were associated with larger index fingertip extensions and lo\r tcstosterone with smaller fingertip extensions. In the total sample (see Table 3), the serum estradiol concentration was negatively correlated with the index fingertip extensions of the right and left hands, while serum testosterone concentration positively correlated with the index fingertip extensions of the right and left hands. That is, higher estradiol levels were associated with lower index fingertip extensions as in the sample of women; lower estradiol levels were associated with greater index fingertip extensions, as in the sample of men. Rzng Fznger (40) Sex dzffrences.-The fingertip extensions of the ring fingers for the right and left hands mere not significantly correlated with body height, as contrasted to the positive correlations for the index fingers (see Table 2). Sex had a main effect on ring fingertip extensions (I;,-,,= 6.3 1, p < .005; q 2 =.02), however, onl) for the left hand ( F,,,, = 6.03, p < .05; q2= .01) and not for the right hand ( F ,,,,=2.45, p > .lo; q2= .00). Controlling for helght changed these results: sex had a significant effect on the ring fingertip extension of the right hand (F,,, =3.94, p < .O5; q Z =.03), the estimated marginal means being 9.4 mm (SE= 0.4) for females and 8.1 mm (SE= 0.3) for males.
Sex had no significant effect on the fingertip extensions of the left hand (F,,,, =3.04, p > .Oj: q2= .03), the estimated marginal ineans being 10.4 mm ( S E = 0.45) for females and 9.3 mm ( S E = 0.35) for males. These results indicate that there were different ring fingertip extension patterns in males and females. TABLE 2 PEARSON
