A comparison of finger 2D:4D by self-report direct measurement and experimenter measurement from photocopy: methodological issues.

The ratio of 2nd and 4th digit length (2D:4D) is sexually dimorphic and may be a correlate of prenatal sex steroids. 2D:4D is often calculated from measurements of photocopies of fingers. However, 2D:4D from photocopies is lower than 2D:4D from direct measurements of the fingers. A new and promising source of 2D:4D measures is self-reports from Internet studies. This necessitates self-report of direct finger measurements and such measurements may be unreliable. In the present study, we compared 2D:4D from self-reported finger lengths measured directly from the fingers (S-R 2D:4D) and experimenter-measured finger lengths from photocopies of the fingers (photo 2D:4D). There were 329 participants (77 men, 252 women) recruited from a first-year undergraduate psychology pool. Compared to photo 2D:4D, (1) S-R 2D:4D tended to include some extreme values; (2) S-R 2D:4D was higher; (3) S-R 2D:4D showed weak similarities which increased when extreme values of S-R 2D:4D were removed; (4) photo 2D:4D and S-R 2D:4D showed lower values for males compared to females but the dimorphism was significant for the former but not for the latter. We conclude that, insofar as S-R 2D:4D has similarities to 2D:4D from Internet studies, the 2D:4D from Internet studies will show extreme values which should be removed, mean 2D:4D will be higher than from photocopy studies, and the sexual dimorphism will be weaker than in photo 2D:4D. We suggest that large samples are necessary in Internet studies of 2D:4D because measurement error will reduce effect sizes.