Heterogeneity's Ruses: Some Surprising Effects of Selection on Population Dynamics

Vaupel, J.W. & Yashin, A.I. (1983). Heterogeneity's Ruses: Some Surprising Effects of Selection on Population Dynamics. IIASA Collaborative Paper. IIASA, Laxenburg, Austria: CP-83-056

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Abstract

Heterogeneity is sometimes used as a synonym for variability or diversity; here it has a narrower meaning of variability with respect to mortality (or with respect to attributes of individuals that affect their mortality). This concept of heterogeneity is closely linked with the concept of selection: a heterogeneous population is one in which there is differential mortality and hence one in which selection is occurring.

Because of the effects of selection, the patterns of mortality (or exit) in a heterogeneous population can differ qualitatively from the patterns of mortality in the constituent sub-populations. These qualitative differences can be surprising; unsuspecting researchers who are not wary of heterogeneity's ruses may fallaciously assume that observed patterns for the population as a whole also hold on the sub-population or individual level. Such incorrect inferences may produce erroneous policy recommendations, because the effect of an intervention usually depends on the behavior and response of individuals. In addition, because rates for homogeneous groups often follow simpler patterns than composite population rates, both theoretical and empirical research may be unnecessarily complicated by failure to recognize the effects of heterogeneity.

The multiplicity of heterogeneity's ruses can be neatly illustrated in the simplest example of a heterogeneous population -- namely, a composite population that consists of two homogeneous sub-populations. It is not difficult to develop models of heterogeneous sub-populations that consist of a very large or infinite number of sub-populations. The ruses illustrated here could have been described in the context of such a model, but for purposes of simplicity and clarity, a focus on the most elementary kind of heterogeneous population seems appropriate. Moreover, it turns out that almost all the distinctive features of heterogeneous populations become apparent as soon as the transition is made from a homogeneous population to a mixed population with two major sub-populations.

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