The evidence comes from a research paper prepared for the Institute for Social and Economic Research by Dr John Ermisch of Essex University entitled 'An economic history of bastardy in England and Wales'
Those women who expect to obtain a significant increase in welfare when they marry suffer a greater long-term cost by having a child while single than women whose marriage prospects are such that they expect to gain little from marriage. Thus, women with poorer marriage prospects are more likely to have children outside marriage. If marriage market prospects are worse for poorer women (e.g. those with low educational attainments), because they can only marry poorer men, we would expect that poorer women would be more likely to have children before marriage, a prediction which is repeatedly confirmed (e.g. Ermisch 2001; Del Bono 2004).Read the whole paper. The proportion of bastards in the population can be reduced to just a few elegant econometric algebraic expressions.