In the olden days, people typically had had as many children as they could afford in order to ensure at least a few of them would survive to adulthood and have children of their own due to high child mortality rates. Moreover, it was the poor who had had an incentive to curb their fertility whereas the rich had lacked such a need due to greater wealth and lower child mortality rates. This changed due to the Industrial Revolution. Standards of living improved and mortality rates fell. People no longer needed to have as many children as before to ensure the propagation of their genes. The link between poverty and child mortality weakened. In addition, societal attitude towards contraception warmed, leading to the negative correlation between intelligence and fertility. Factors linked to general intelligence, such as socioeconomic status and educational attainment, were also found to be negatively correlated with fertility starting from the nineteenth century.