The disposibility theory of aging is one of numerous evolutionary theories of aging that seek to explain why aging exists and is near universal across species. In this case, aging is viewed as the inevitable result of trade-offs between resources allocated to reproduction versus resources allocated to tissue maintenance. Like near all evolutionary theories, and particularly those relating to aging, the models and the science are much debated.

Since there is some variation between individuals within a species, one should expect to find a distribution of outcomes for any given trade-off when comparing large numbers of individuals of a given species. In this case, for this view of the origin of aging, we should expect to see that greater reproductive success correlates with a worse outcome in later life. Meaning a faster decline, more age-related disease, and a shorter life expectancy.

In today’s open access paper, researchers compare parity (number of children carried to term) with later frailty in a human population. They indeed observe that more births tends to correlate with greater age-related frailty. The challenge with human data is that one can always come up with other plausible explanations for this effect, completely unrelated to fundamental biology. That the effect is similar in men and women somewhat sabotages any thoughts of a biological or physiological cost to childbirth as a dominant mechanism, for example.

Frailty: A cost incurred by reproduction?

The disposability theory of ageing proposes that investing in reproduction, at the cost of somatic maintenance, leads to senescence. In humans, the theory predicts that those with more children will have shorter lives. Researchers used a historical dataset from the British aristocracy to demonstrate that females with the longest life span had fewer children relative to the whole sample. Indeed, almost 50% of females who lived to 80 years and over were childless. A similar relationship between parity and longevity was found in males. The paper was criticized in the literature, particularly with regards to the quality of the data. Despite a sustained research effort and strong theoretical expectations, evidence to support a reproduction-longevity trade-off in humans is not strong. Studies of historical and contemporary cohorts have not found a consistent association between parity and longevity – no association, as well as positive and negative associations, have all been reported.

Most studies to date have tested evolutionary theories of senescence by focusing on the relationship between parity and survival (usually measured in terms of longevity). However, it is possible that survival is too crude a measure of senescence and, as a result, the ‘real’ cost incurred by reproduction has not been elucidated. Whilst studies have examined other health outcomes, such as physical, functional and cognitive impairment, self-rated health and limiting long-term illnesses in older males and females, findings have not been consistent. Examining the relationships between parity and individual domains of health may not be the best methodology to address the hypothesis because impairment profiles vary significantly in the older adult population and measures of individual domains do not capture all adults with poor health. ‘Frailty’, on the other hand, is a multidimensional measure of health status that may help to better define the long-term consequences (whether they be harms or benefits) of human reproduction.

The aims of this study were to examine the cross-sectional relationship between parity and later life frailty (represented by the Frailty Index) and to explore whether this relationship is influenced by sex. Data from the English Longitudinal Study of Ageing (ELSA) were used to test two key hypotheses: firstly, that higher parity is associated with greater frailty, indicating a ‘parity-frailty trade-off’ and secondly, that sex differences in frailty are greater at higher parities than at lower parities due to sex differences in the physiological costs of childbearing.

We found that the most parous adults were the most frail, providing weak evidence for a ‘parity-frailty trade-off’. The relationship between parity and frailty was similar for both sexes, and thus the results suggest that behavioral and social factors associated with rearing many children may be more relevant to the parity-frailty relationship than the physiological burden of childbearing. Parity-frailty trade-off may manifest in older males and females with high parity due to economic strain, disruption of occupational attainment, and psychological stress. In addition, high parity may negatively influence lifestyle habits such as dietary choices and physical activity in both sexes. These behavioural factors increase the risk of obesity and its metabolic complications, which in turn, increase the risk of frailty.

An alternative theory is that selection effects confound the relationship between high parity and frailty. For example, lower levels of education level are associated with particular reproductive characteristics, such as early parenthood and higher overall parity, as well as later life frailty. However, in this study, education was not found to have a significant impact on the parity-frailty relationship.