Sure, I think I can find studies that affirms my biases too if I really want to.
Personally, I don't believe that parents are so miserable and childless people are so happy. Maybe at some stage in life that can be measured to be objectively true, but longterm, dude, I think the parents win the happiness lottery if the childless people choose to never move on from their 20s and grow up and take part in their community. I invest time and money into my nieces and nephews and into my friends' kids too. Because they are family and I care about them and their parents and it gives me joy to know that I'm a part of something either directly or indirectly, depending on circumstances. I do it because I know that there is also a day after tomorrow where I would become alone and forgotten and have no one to lean on if I don't contribute and invest anything into the future, which in this case it other people's children.
At some point, we have to remember that the world is bigger than ourselves and if we only invest in ourselves our whole lives, we will end up very very very alone.