Digit ratio (2D:4D) and month of birth: A link to the solstitial-melatonin-testosterone effect

Abstract

Background: Digit ratio (2D:4D) is a sexually dimorphic trait and its determination in utero is influenced by testosterone. The solstitial-melatonin-testosterone hypothesis posits that melatonin inhibits the production of foetal testosterone and melatonin levels are at their lowest in months when light levels are high.

Aims: We test the relationship between 2D:4D, month-of-birth and light levels.

Study design: We recruited participants whose year of birth was spread across the 20th Century.

Subjects: 323 Polish men and women.

Outcome measures: Finger lengths, month-of-birth, mean daylight hours per month in and around Poznan, Poland.

Results: Our sample was born between 1907 and 1997. In comparison to late-Spring births, late-Autumn births had low right-left 2D:4D (high prenatal testosterone). Regarding light levels, there were significant relationships between low right 2D:4D and right-left 2D:4D (high prenatal testosterone) and long days at the end of the 1st trimester. These relationships were strongest for participants born in the first half of the 20th Century.

Conclusions: Participants born in the late-Autumn and who experienced long days in the 2nd and 3rd prenatal months had low 2D:4D. The effects were strongest for early 20th Century births where photoperiods would be less disrupted by artificial light.

Keywords: Digit ratio; Month-of-birth; Solstitial-melatonin; Testosterone.