  Does bearing children shorten a woman's life expectancy? Several demographic
studies, historic and current, have found no such effect. But the Caerphilly
cohort study is far the most prominent and frequently-cited, and it answers in
the affirmative. Why has this study found an effect that others fail to see?
Their analysis is based on Poisson regression, a statistical technique that is
accurate only if the underlying data are Poisson distributed. But the
distribution of the number of children born to women in the Caerphilly database
departs strongly from Poisson at the high end. This makes the result overly
sensitive to a handful of women with 15 children or more who lived before 1700.
When these 5 women are removed from a database of more than 2,900, the Poisson
regression no longer shows a significant result. Bi-linear regression relating
life span to fertility and date of birth results in a positive coefficient for
fertility.
