You are correct to note that the real problem is that the poor are more likely to be bad people in general, and that they will drag down other poor folks. Poor people are likely to be bad parents (as you note) and are far more likely than non-poor people to engage in violence against friends, relatives and neighbors. No one disputes this.
The point is that it's not economics keeping people from the sciences.
(Here's some numbers on violence among poor people as compared to non-poor people: http://www.bjs.gov/content/pub/pdf/hpnvv0812.pdf )