They found on average a 19% difference between the highest and lowest social strata in the risk of dying in each country and about half of the difference was due to the risk of being killed by smoking.

BBC: NEWS | Health | Smoking 'accounts for health gap'

But the word "caste" is routinely used by government experts to refer to social strata in underprivileged Muslim communities.

WSJ: India Classifies More Castes as 'Backward' In Affirmative-Action Bid to Help the Poor

Society being composed of social strata a family will find its children in competition with children from families in roughly the same social-economic stratum, with roughly the same total amount of family income to invest in children.

FORBES: Demographic Decline and Investment Intensity in Children -- Observations on China and Japan