Illegitimacy in a historic, single community at Penrith, Cumbria (1557–1812), has been studied using aggregative analysis, family reconstitution and time series analysis. This population was living under extreme conditions of hardship. Long, medium and short wavelength cycles in the rate of illegitimacy have been identified by time series analysis; each represents a different response to social and economic pressures. In a complex interaction of events, the peaks of the cycles in wheat prices were associated with rises in adult mortality which promoted an influx of migrants and a concomitant rise in illegitimacy. The association between immigration and illegitimacy was particularly noticeable after the mortality crises of the late sixteenth and early seventeenth centuries. Children of immigrant families also tended to produce illegitimate offspring. Native and immigrant families responded differently to extrinsic fluctuations, and variations in their reproductive behaviour were probably related to access to resources.