The analysis of the three broad categories of childlessness (natural sterility, social sterility, voluntary childlessness) outlined above helps to understand how it has changed over the last century in the United States. At the end of the 19th century, income and education levels were low. This made levels of social sterility very high. In addition to the causes mentioned above, the Spanish Influenza epidemics meant that pregnant women who were infected were particularly vulnerable to miscarriages. The Great Depression also impoverished these generations, for whom voluntary childlessness was almost absent. On the whole, the rates of childlessness for married women born between 1871 and 1915 fluctuated between 15 and 20 percent. The rise in both education and overall income allowed subsequent generations to escape from situations where couples were “constrained” from having children, and rates of childlessness began to fall. Over time, the nature of childlessness changed, becoming more and more the chosen outcome of some educated women. A low level of childlessness of 7% was achieved by the generation of the baby boom. It started to rise again for the subsequent generations, with 12 percent of women born in 1964-68 remaining childless. Social causes of childlessness have now completely disappeared for women in union. This is however not true for single women, who are usually poorer, for whom social sterility still exists. Due to polyandry and hypergamy, twice as many men in history have been childless than women.
Childlessness, which may be by choice or circumstance, is distinguished from voluntary childlessness , or being "childfree", which is voluntarily having no children.