You could research it yourself on Google Scholar if you are really interested:
here’s an article from 1949. I’m sure there must be other studies!
On the basis of these findings it finally is justified to conclude that the number of only children among persons with self-destructive tendencies does not differ significantly from the expected number as deduced from data available with respect to the general population. Evidently, only children are neither more likely nor less likely to commit suicide than are any other persons, including those who had an opportunity to perpetrate sibling rivalry or to develop identification mechanisms in intimate sibship relations.
Suicide in Twins and Only Children FRANZ J. KALLMANN, JOSEPH DE PORTE, ELIZABETH DE PORTE AND LISSY FEINGOLD
The explanation I know about why homosexual genes aren’t weeded out, if that homosexuals exhibit higher altruism. Because of this, they are more disposed to help and support their siblings—such as participating domestic duties like child rearing—in ways they help spread copies of their own genes via their siblings. After all, your siblings share 1⁄4 of your DNA.
See Edward O. Wilson “On Human Nature” chapter on altruism.
You could research it yourself on Google Scholar if you are really interested:
here’s an article from 1949. I’m sure there must be other studies!
Suicide in Twins and Only Children
FRANZ J. KALLMANN, JOSEPH DE PORTE, ELIZABETH DE PORTE
AND LISSY FEINGOLD
The explanation I know about why homosexual genes aren’t weeded out, if that homosexuals exhibit higher altruism. Because of this, they are more disposed to help and support their siblings—such as participating domestic duties like child rearing—in ways they help spread copies of their own genes via their siblings. After all, your siblings share 1⁄4 of your DNA.
See Edward O. Wilson “On Human Nature” chapter on altruism.