To what extent is the number of children people choose to have in developed nations heritable (in the genetic sense)?
If it is heritable, this is most likely the largest determiner of fitness outcomes, as the great majority of children survive to adulthood, no matter how many siblings they have. This implies potentially rapid Darwinian selection for those who choose larger families. Over the course of a few centuries this selection could have important impacts on human demography and population. Given an estimate of heritability, this effect could be quantified.
The old twins/siblings separated at birth databases?
Ethical considerations: many
Likelihood I will actually get around to this: