We use data collected by the Cambridge Group to investigate and explain differences in fertility by socio-economic group in pre-industrial England. We find, in line with results presented by Greg Clark, that wealthier groups did indeed have higher fertility until the 1700s. We demonstrate that this had to do with earlier age at marriage for women. We then turn to the likely social and economic impact of this, considering Clark's hypothesis that ‘middle-class values’ spread through English society prior to the Industrial Revolution. Through the construction of social mobility tables, we demonstrate that the children of the rich were indeed spreading through society, but they were small in number relative to poorer sections of society, and moreover the children of the poor were also entering the middle classes.