The problem with those studies has always been that there is no objective way to measure happiness. What they're usually measuring is stress and energy levels during a particular period. The 1st year with a baby is tiring and stressful. You do have to compromise a lot.
Also, every single one of those studies which I've read had the caveat that most of the parents said it was worth it even with the pressures. That should tell you something about the problems with measuring this sort of thing.
Personally, my daughter just turned one a few weeks ago. This year was tiring, stressful, full of compromises, and yet, as with some of these other parents, worth it to me. The things I compromised on mostly have turned you to be things I didn't really need in the first place. Sure, there are times when it would be nice to have a babysitter for the evening to go out with friends, and I'm less inclined to come home hung over now since I don't want to be the one up the next morning with my daughter, but some of that will come back as she gets older. When all is said and done, a lot of this is temporary until the child grows up a bit more.
You can argue it's a form of Stockholm syndrome or some sort of self-delusion, but what difference does it make if I still tell you I'm happy now?