Causes of an extremely high local twinning rate.

The Romanian village Eftimie Murgu, until recently quite endogamous, shows a remarkably high total twinning frequency of 20% between 1927 and 1977. Some other demographic properties are unusual too: the secular development of twinning is not so much characterized by a steady frequency decline as by a sharp and temporary depression after World War II. The distribution of twins on maternal age and parity is not only characterized by an increase with age/parity but there is also a peak in the lowest ages/parities. The well-known higher fertility of twins' mothers is confirmed. In order to check the possible genetic determination of the high twin frequency, pedigrees of six to eight generations were reconstructed for every twin; moreover, twin frequencies of other isolated Romanian villages were examined for comparison. These data suggest that genetics is an important cause of twin births.