A twin study of skin reflectance.

Skin colour has been measured by reflectance spectrophotometry on 134 pairs of twins at three sites, forehead, forearm and upper arm, each at three wavelengths, 425, 545 and 685 nm. Tanning is measured most reliably at 685 nm and at this wavelength the heritability is high at the least exposed upper arm site, intermediate on the forearm, while on the forehead variation is entirely environmentally determined. The same gradient is observed, through less strikingly at 545 nm, but at 425 nm, where haemoglobin is reflecting most of the light, the degree of genetic determination is the same at all sites.