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

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.

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

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

323 Polish men and women.

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

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.

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.