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

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.