From the initial Maternity Survey of 13,687, a sample of 5,362 singleton, living, babies born to married parents was selected for follow-up. 

This sample was stratified to include all single births to married women with husbands in non-manual and agricultural employment and one in four of all comparable births to women with husbands in manual employment.

 We are still in contact with over 80% of the original surviving sample living in mainland Britain.