The lifespansof parents and children appearonly weakly related,even though parents affect their children’s longevity through both genetic and environmental influences. These influences can be summarizedas a correlation betweenparents’and children’s frailty. It isshownthat even ifchildren perfectly inherit their frailty from their parents, parents’ life spans explain little of the variance in children’s life spans, because the variance in life expectancies among people with different frailties is small compared with the variancein life spansamongpeopleat the same leveloffrailty. Byinterpreting frailty as a relative risk in a proportional-hazard model, longevity as a duration or waiting time, and inheritance as an invariance in relative risk over time, one can extend this result to repeatable events involving fertility, migration, marriage, unemployment, and so forth.

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