See map below (hope it appears). The USA is the world leader in the percent of single-parent homes. Of course, there is a strong causal relationship between a high percent and income inequality, crime, school dropouts and low test scores.
Note: The high percent for Denmark is misleading, because my understanding is that fathers in Denmark and other Scandinavian countries stay close to their kids and support them emotionally and financially, even if they aren't married to mom and don't live with the family. Yes, there is also a strong welfare state in those countries, but my further understanding is that the culture is such that citizens tend not to abuse the system, although this is beginning to change as the countries become less homogeneous as a result of immigration.