It is a truth universally acknowledged that although men tend to have better health in old age, women live longer lives. Here, we briefly review the biological, social and behavioural factors that may contribute to women's greater longevity. We consider in particular factors that might result in a greater frailty burden in women, focusing on frailty being measured by a Frailty Index. The Frailty Index represents the burden of health deficits, expressed for an individual as the proportion of deficits present - from 0 (no deficits) to 1.0 (the theoretical maximum, if all deficits were expressed). A greater frailty burden in women might first represent a male "fitness-frailty pleiotropy", resulting in men having lower physiological reserves in old age so that health deficits are more lethal. In short, the price of more optimal physiological functioning during youth is a lower threshold for system failure in old age. Conversely, a female "fertility-frailty pleiotropy" might result in greater physiological reserves in women. Child birth and child rearing necessitate high levels of energetic and nutritional investment: women who have children live shorter lives. Women currently are limiting the number of children they bear and their life expectancies may be longer than predicted by evolutionary design. Third, though the Frailty Index captures physical, cognitive and psychological vulnerability, it may not include all factors that impact life expectancy in older people; these factors may be present more in men than in women. While these hypotheses seek to explain how frailty impacts men and women in different ways, there is clearly much to be done to understand frailty in older people.
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