…a principal reason for greater income volatility is both simple and benign–motherhood. In the 1970s, a minority of mothers were in the workforce and their pay was relatively low. By the 1990s, a majority of mothers were in the workforce and their pay was much higher. Because women today have a much more prominent role in the economy, their movements in and out of the workforce to take care of children are having bigger impacts on income volatility. When mothers re-enter the workforce, family incomes increase. This also counts as income volatility.
That is from The New Rules Economy. Have any of you seen more on the topic?