Twinning trend in Escazú, Costa Rica, 1851-1901.

The cross-cultural study of secular trends in twinning has been of interest to students of human biology. Although recent vital statistics are likely to be more reliable, older vital records allow a long-term study of twinning trends and of the effect of epidemics on twin incidence. Here, I report the results of an investigation of the secular trend of twin maternities from 1851 to 1901 in Escazú, Costa Rica. The research also investigates whether epidemics affected twin incidence in a consistent manner. Because the church records did not state the mother's age, it was not possible to standardize the raw twinning rate for maternal age. Thus the moving averages of the raw rates for every 33 years (the average length of a generation) were computed to minimize the effect of changing average maternal age on twin incidence. No consistent response to epidemics is apparent in the twinning rates. However, both the raw twinning rates and their moving averages indicate that the frequency of twinning decreased in Escazú during the second part of the nineteenth century. A decline in the twinning rate in other populations is usually attributed to environmental changes caused by the process of industrialization. However, the decline in the twin maternity incidence in Escazú could not be the result of such an environment because the population was rural and nonindustrialized. This study indicates that a decline in twin maternities may result from more diverse causes than was previously thought.