Here’s a go: There is a Professor of Psychology at Harvard University who is investigating Happiness. He has a website called “Track Your Happiness” where you can get involved yourself if you wish.
There is an article about him and his work in the ‘New Scientist’ of 16th April 2011 called ‘In Pursuit of Happiness’.
In that article they describe a list of things that cause happiness or don’t and they say that the list is pretty much what you’d expect it to be, after all their investigation (which is ongoing, and which you can be part of) – but they find one surprising thing: Children. It turns out having children causes a “… small but reliably negative impact on happiness.”
This means you can rely on it. You can safely predict it. Children will diminish your happiness.
How do you like them apples?