having grown up as an only child, I felt very strongly about have a sibling for my first born. I believe you learn important lessons about cooperation and conflict management from having siblings.
Also, I wonder if the study looked at the years after the kids are grown? I had 5 kids and although that was a few too many in terms of financial stress, etc., when they were growing up... the rewards now that they are all grown are wonderful.
Sara
"I never give them hell. I just tell the truth and they think it's hell." - Harry Truman