Use of an “ Online Ruler ” to Self ‐ Measure Digit Ratio

The ratio between 2nd (index) and 4th (ring) finger lengths (2D:4D) has been proposed to be negatively correlated to prenatal testosterone exposure in humans (Manning, 2002). Evidence for this phenomenon has come from findings of differences between males and females, masculinized 2D:4D in individuals with congenital adrenal hyperplasia (a condition which produces elevated prenatal testosterone), experimental manipulations with animals, 2D:4D being related to receptivity to testosterone (androgen receptors), masculinized 2D:4D in females with male co-twins, and an association between 2D:4D and hormone levels taken from prenatal fluid (see Manning, Churchill, & Peters, 2007 for a review). Initial studies that assessed 2D:4D used photocopies of the hand that were subsequently measured by researchers. More recently, attention has been given to participant self-reported 2D:4D. Caswell and Manning (in press) found moderate correlations between participant self-reported and experimenter measured 2D:4D from photocopies when outlier self-reported values were excluded. From a large Internet study using self-reported 2D:4D, Manning et al. (2007) also found significant effects with lower effect sizes in sex differences than in previous studies. These studies show that self-reported ratios are an acceptable measure of 2D:4D when large samples are used to compensate for the increased error resulting from using non-trained measurers. Furthermore, Manning et al. concluded that direct measurement of finger length is preferable to using photocopies which may not approximate actual 2D:4D as well as first thought. In designing an online questionnaire that assessed 2D:4D as well as a number of other gender-and sexuality-related variables I was aware that many participants would not have access to a ruler, so I decided to trial giving them the option of using an " online ruler " which is simply the image of a ruler in a pop-up browser window. This ruler can not accurately measure finger lengths because the size of the picture, and thus the scale of the ruler varies depending on the screen resolution settings of the computer. However, provided both fingers