This turns out to be a significant topic of study. See here and here. Most of this is about moms with infant/toddlers. From my perspective, happiness is irrelevant. There are moments along the entire spectrum of happiness/non-happiness. Parenting simply has to be done, no matter how happy it makes one. People who have the luxury to abdicate may have some theshold below which they feel free to leave. The rest of us are better off not thinking along this axis at all. People thinking about becoming parents should think a lot about it, however.
Parental happiness January 26, 2014