Bill Gardner says there are two theories:
The stupid people theory:
[M]any poor people have poor cognitive skills — that’s a reason why they are poor — and people with poor cognitive skills smoke, drink too much, eat bad food, and so on. These behaviors lead to poor health…SES is strongly associated with health and with many health behaviors. David Cutler has data showing that intellectual abilities are associated with health behaviors. Linda Gottfredson reports data showing that intelligence predicts both health and longevity.
The scarcity theory:
Sendihl Mullainathan and Eldar Shafir present extensive experimental data showing that the experience of scarcity deeply affects our ability to think:
Poverty itself taxes the mind… poverty reduces fluid intelligence and executive control… this suggests a major twist in the debate over the cognitive capacity of the poor. We would argue that the poor have lower effective cognitive capacity than those who are well off. This is not because they are less capable, but rather because part of their mind is captured by scarcity.
My theory: The welfare state encourages people to be poor and subsidizes all of the unhealthy behaviors associated with being poor.