Below, Corrections depicts the proportion of women by marital status and labor force status over time from the Current Population Survey (click to enlarge).
What could cause this? Why are married women working so much more? It can't be anything special about being married--nonmarried and married women show the same pattern. Women in general are working more.
Even though we know it isn't anything to do with martial status, we then depict the proportion of married women by labor force status over time (click to enlarge). Women used to work about 35% of time (other sources give 35% extending back further) to around 60% of the time, an increase of 35% (doubling the proportion of women working).
Corrections then offers the proportion of women by martial status, educational status, and labor force status over time. One trend dominates the pattern: women are becoming more educated, so both red lines decline while both blue increase (click to enlarge).
This is depicted more clearly below (click to enlarge):
Instead, we can normalize by the population of all women within an educational category (click to enlarge):
And we can normalize that to the proportion that was working in 1965, to see how much these proportions change over time (click to enlarge):