In the U.S., and probably in other countries as well, there’s been an increase in the number of single parent families. Most of the time, that means a single mother, divorced or unmarried, or with a husband in prison, and raising one or several children on her own. As a result:
The percentage of children living with one parent has doubled since 1970, from 12 percent to more than 26 percent in 2004. (source)
There are about 13.7 million single parents in the United States today, and those parents are responsible for raising 21.8 million children. 84% of those single parents are mothers.
Single mothers often earn relatively lows wages, partly because they can’t afford to work long hours. Combine that with the fact that they have higher per person expenses (heating a house costs just as much for a two parent family as for a single parent family) and the fact that women in general have lower wages, and you have a recipe for inequality.
However, the growth in the number of single parent families in the U.S. flattened when income inequality continued to increase. So, family structure may be a good although partial explanation of poverty levels, but not necessarily of inequality. There must be other causes, some of which are discussed here.