Methods for the identification of Lexian, Poisson and Markovian variations in the secondary sex ratio.

One of the problems arising in the identification of the type of process generating the secondary sex ratio in human and other populations is that of 'spurious contagion'. A model is presented which allows the simultaneous estimation of the effects of (i) variation in the biological predisposition of couples to have children of a particular sex, (ii) variation in this predisposition with birth order, (iii) a Markov association between the sexes of adjacent sibs as the result of in utero interaction, and (iv) the socio-psychological predisposition of couples to limit families once they have obtain a desirable composition. Restricted forms of the model allow one to test for the presence of each of these effects whilst controlling for the others. The model is applied to data from the United States and Finland.