Digit ratio (2D:4D) and body mass index in the BBC Internet Study: prenatal sex steroids and a Trivers–Willard effect on body composition

Abstract

Digit ratio – a putative measure of prenatal sex steroids – may be related to body mass index (BMI). However, reports of correlations between 2D:4D and BMI have yielded mixed results with some studies showing no relationship while others have reported positive associations in men or women only. This study considers associations between self-reported 2D:4D and BMI in a large online survey (i.e. the BBC Internet Study). At the individual level, there was a weak positive association between 2D:4D and BMI in both sexes with greater effect sizes in women. Body mass index was positively related to age and negatively related to parental income; however, the relationship between 2D:4D and BMI was independent of both variables. At the national level, mean 2D:4D per country showed positive associations with mean national BMI but those correlations were restricted to females. It is concluded that BMI is positively related to low prenatal testosterone and high prenatal oestrogen. Parental income inequality may influence both prenatal sex steroids (through a ‘Trivers–Willard’ effect) and BMI such that increases in inequality result in reductions in prenatal testosterone and increases in BMI at the individual and national level.

The 2D:4D index is associated with the development of excess body weight in adults, but not with the rate of weight loss following bariatric surgery

“…On the other hand, Fink et al found that BMI in men (but not in women) is positively related to the digit ratio, but it remains significant only for the left hand 36 . Contrary to this report, Manning et al indicate that 2D:4D is positively associated with BMI, but only in women 37 .…”

“…This is indicated by higher 2D:4D values of the right and left hand in people with obesity than in people with normal body weight. This result is consistent with the reports of many other authors 32 – 34 , 36 , 37 and indicates a new area of activity in the prevention of this disease. At the same time, it seems that the 2D:4D index (especially of the right hand) may be a useful factor in early prediction of the risk of excessive body weight development in humans.…”

“…There are also studies that do not show the existence of such a relationship in young women 37 , 38 as well as young women and men 39 – 42 . On the other hand, others reveal an inverse correlation between 2D:4D and BMI 43 .…”

Association of prenatal sex steroid exposure estimated by the digit ratio (2D:4D) with birth weight, BMI and muscle strength in 6- to 13-year-old Polish children

“…2D:4D R values were associated with higher body weight and BMI in a group of 320 males aged 3-22 [15]. Manning et al finding in adults showed that 2D:4D values (R & L) were positively related to measures of BMI in a very large sample (255,116 participants) [13]. Our findings in children study group are further evidence suggesting the positive relationship between 2D:4D and BMI is influenced prenatally.…”

“…To date few studies have analyzed the 2D:4D digit ratio and body parameters or proportions in adults, with the findings remaining inconclusive [8][9][10][11][12]. However, recently published study conducted by Manning et al showed that 2D:4D is positively related to BMI in a very large sample of adults (255,116 participants) [13]. This indicates the need to investigate whether and how prenatal sex steroid exposure affects developmental processes from prenatal stages through childhood and adolescence to adulthood.…”