We can't resist using birth order to explain our personalities and it seems intuitive that sibling-less children might experience the pros (all the parents' attention and resources) and cons (selfishness, loneliness, and a reliance on parents) of never having someone to share with. Lucky for them, modern studies suggest that only children only see increased advantages.
Following a definitive 1896 study, conventional wisdom has held that only children will be different (not in a good way) than people with siblings. But an educational psychologist from Texas, working with only children since the 1970s, has found that only children do not show any measurable difference except that they score higher when it comes to intelligence and achievement, as do firstborns and people who only have one sibling. So I guess we should stop worrying about only children and start feeling reluctant about having more than two kids — at least that would be cheaper.