This fits to something Dan Savage (not a scientist but someone worth listening to on matters of family relationships) said:
Kids are like heroin, a little heroin addiction. When it's bad, you've never been so miserable, but when it's good you've never been so high.
There is a lot of research on this topic. I know later studies contradict earlier studies, but I'm not sure who to believe here. Wondering if anyone can help.
In case it isn't clear, the basic question is the effect of having children on happiness.