Differential twin mortality indicates a correlation between age and parental effort in humans

A two-resource model of terminal investment

The most widely known theoretical basis for the hypothesis of terminal investment is the classic model by George C. Williams (1966). Although this model predicts that reproductive effort (i.e. the proportion of available resources devoted to reproduction) increases with decreasing reproductive value, it implies that reproductive allocation in absolute terms should remain stable. This contrasts with the empirical evidence on terminal investment reported to date: the vast majority of positive case studies report an increase in some aspect of reproductive allocation in absolute terms. Also, a substantial number of studies have failed to record terminal investment, despite expectations. Here, I present a simple conceptual model which explains such results. I argue that to explain terminal investment, an organism's reproductive capacity must not be considered as a common pool of resources (often described by the term 'reproductive value'), but as a set of different resources which are not easily convertible to each other, and should be exhausted in balance. Thus, if one resource accidentally decreases, in response, the others must be expended at higher rate. To test this model, each reproductive allocation should be measured in a more specific currency (or currencies) than traditional 'reproductive effort'. The model is consistent with both the positive and the negative case reports on terminal investment.

Twinning in humans: maternal heterogeneity in reproduction and survival

While humans usually give birth to singletons, dizygotic twinning occurs at low rates in all populations worldwide. We evaluate two hypotheses that have differing expectations about the effects of bearing twins on maternal lifetime reproduction and survival. The maternal depletion hypothesis argues that mothers of twins will suffer negative outcomes owing to the higher physiological costs associated with bearing multiples. Alternatively, twinning, while costly, may indicate mothers with a greater capacity to bear that cost. Drawing from the vast natural fertility data in the Utah Population Database, we compared the reproductive and survival events of 4603 mothers who bore twins and 54 183 who had not. These mothers were born between 1807 and 1899, lived at least to the age of 50 years and married once to men who were alive when their wives were 50. Results from proportional hazards and regression analyses are consistent with the second hypothesis. Mothers of twins exhibit lower postmenopausal mortality, shorter average inter-birth intervals, later ages at last birth and higher lifetime fertility than their singleton-only bearing counterparts. From the largest historical sample of twinning mothers yet published, we conclude that bearing twins is more likely for those with a robust phenotype and is a useful index of maternal heterogeneity.

Impairment in family caregiving: a biological perspective

This paper focuses on patterns in the care of children and dependent elders, providing a new perspective to understand impairment in family caregiving. One concept from evolutionary theory, kin selection, has promise for providing a broad framework for understanding patterns of family caregiving. We present an application of the evolutionary model to the issue of family care decisions in the face of potential conflicts of interest. Specifically, a mathematical biological model, which was developed to understand self-preservation motivation, is applied to the kin investment decision. Using the evolutionary model, the main predictors of investment would be relatedness, reproductive value, and expected benefit from the care invested. The biological model accounts very simply for decisions between caring for an elder parent or caring for your own child, and for decisions by two daughters (one with a child, one childless) to care for an elder parent. The factors that form the basis of an evolutionary model (relatedness, reproductive value, and benefit from care) have not been widely investigated in family care studies, however, and this absence reveals new directions for future research.