Does having children make you happier?
Depends on what you mean by happiness. If you check in with people at random intervals and ask them what they are doing and how they feel, you find that when they are with their children they are often in bad moods. (Yes, people agree to participate in that kind of study all the time, although you have to wonder if such people are really a representative sample of humanity.) So if you are looking at people's moods moment to moment, it may well be that having children is a negative factor.
But what if by "happiness" you mean something else, like, how you feel about your life when you reflect on it? A new study from a team of psychologists finds that when you ask how satisfied people are with their lives, parents are on average more satisfied than their childless peers. The effect is stronger for fathers than mothers, perhaps because "the pleasures of parenthood may be offset by the surge in responsibility and housework that arrives with motherhood." The effect is also much stronger for parents who have children older in life, and quite small for very young parents.
The researchers note that these are correlations, not necessarily causes. It occurs to me that marriage has been shown to have a big impact on happiness, so if more married than single people have children, that could skew the data. Religion is another possible factor, since religious people tend to be happier, and I certainly have the impression that they have more children.
But why these contradictory findings? I would say that this points to the big problem with happiness research as a field. The things that make us feel good about ourselves and our lives, looking backward, are often frustrating and exhausting while we are living through them. A life devoted to moment-to-moment pleasure may add up to a miserable sense of failure, whereas a life of accomplishment may involve thousands of hours of hard, unfun work. Or, we might put ourselves through thousands of hours of hard, unfun work and end up thinking it wasn't worth it and we should have played video games instead.
I don't think happiness research can give us a detailed plan for life. People differ, and even if something is true on average it may not be true for you. But there are some regularities that come out of happiness research: connections to other people are key, having experiences matters more than accumulating things, and a good marriage is worth more than all the money in the world.