Does Finger Fat Produce Sex Differences in Second to Fourth Digit Ratios?

The year 2009 marks the 50th anniversary of the publication of Phoenix, Goy, Gerall, and Young’s (1959) landmark study in Endocrinology (1). This study provided the basis for the Organizational Hypothesis, the notion that exposure to hormones, thought to be androgens, during sensitive periods permanently altered the structure and function of the nervous system (2). The 50 yr since its publication have produced overwhelming support for the Organizational Hypothesis, and it is now a cornerstone of our understanding of how behavioral sex differences differentiate during development. Although the organizing actions of hormones on the nervous system can produce behavioral changes, they may not leave external physical markers of their actions, such as altered genital anatomy. This lack of external physical evidence has resulted in great interest in discovering easily observed physical proxies for exposure to differing levels of organizing hormones, particularly androgens, during development. One of the most attractive candidate proxies has been the ratio of the lengths of the index (2D) and ring fingers (4D), known as the 2D:4D digit ratio, or simply digit ratio (3). This ratio, which typically is significantly smaller in men than in women, has been argued to decrease with higher and increase with lower levels of prenatal androgen. In fact, with little supporting evidence (4), digit ratios are often referred to as “a biomarker for prenatal levels of testosterone” (5), reflecting the view that variations in prenatal testosterone produce variation in digit ratios. Definitive evidence that digit ratios reflect prenatal exposure to androgens has not been presented. Thus, the study by Berenbaum and colleagues (4) in this issue is of particular significance.