1 Early History

To our knowledge, the first use of twin resemblance as a means of resolving alternative hypotheses about the causes of human individual differences appears in 426 A.D. by Augustine of Hippo in Book V of the City of God. Augustine argued that since twins, who were highly correlated in their times of birth, nevertheless had such discrepant life histories, there was little empirical support for planetary influence on human destiny. For Augustine's purpose, it was sufficient that at least some twin pairs showed markedly different life histories, despite being born at the same time. To go beyond testing the astrological hypothesis and use twins to answer the nature-nurture question required recognition of the fact that there are two types of twins, identical and fraternal, and some way of distinguishing between them.