So these British researchers concluded that kids who are only children are happier.
Guess what? All the adults I know who were only children have sworn they would never inflict that on their children if they had any choice in the matter. ALL of them wish they had had siblings. All of them said they missed out on learning important things that made growing up much harder for not having siblings.
Perhaps the British researchers should have concluded that children with alert parents who don't allow their children to mistreat each other are happier. I don't think the answer to kids bullying each other in the home is to have only one child. I think the better answer would be to teach your children, nip bad habits in the bud, and be alert and attentive as parents. And--dare I say it?--this is easier if you have one parent who is a stay-at-home parent, so that SOMEONE is paying attention all the time to how the kids are treating each other, and there is no handing off of children (and their discipline and habits) to various caregivers who all have different ideas and ideals.
Just one more example of valid data leading to the wrong conclusion.
Bottom line: if having a lot of kids was bad for kids or for parents, God wouldn't have asked us to do it, much less made it a part of our marriage covenants.