by Brendan Zietsch
Life history theory has been promoted as a unifying framework for understanding trait covariation in the social sciences. Underlying this approach is the assumption that patterns of trait covariation across species, and the Darwinian principles thought to underlie those patterns, can be extrapolated to trait covariation across individuals within human populations. In particular, it is assumed that a ‘fast-slow’ continuum of life history ‘strategies’, observed across species, should be observed across individuals within human populations – not only with regard to traditional life history traits but practically all individual differences, including personality and psychopathology.
In my paper with Morgan Sidari we argue that these assumptions are not justified. The processes that create trait covariation across species and across individuals are fundamentally different, and so the two types of covariation need not be parallel or even related. This fundamental problem undermines a great deal of research in this burgeoning field, and warrants a rethink moving forward.
To illustrate the point, we use the simple example of body height. Humans are taller than rabbits because of their different genes. Individual humans also differ in height largely because of their different genes. But the processes that create genetic height differences between rabbits and humans (i.e. Darwinian evolution) are nothing like the processes that create genetic height differences between individual humans. The fact I am taller than a rabbit is primarily due to our different histories of Darwinian evolution over countless generations of different selective pressures; the fact I am taller than my friend or my brother has little (or nothing, in the case of my brother) to do with our different histories of Darwinian evolution, but rather my chance inheritance of more “tall alleles” compared to them. As with all individuals of European ancestry, many of my friends and I share nearly the same set of ancestors who were alive 1,000 years ago (and likely many ancestors more recent than that) – a very short period in evolutionary time. Therefore, the reason my friends and I are of different heights is largely independent of our different histories of Darwinian evolution. We are different heights mostly for the same reason my brother and I (who share all our ancestors) are of different heights – that is, the random shuffling process of Mendelian segregation during meiosis gave us different combinations of our ancestors’ genetic material. Overall, this example highlights why we cannot straightforwardly transfer the same evolutionary principles from the explanation of human-rabbit differences to human-human differences.
“Darwinian phenotype-environment matching at the species level and plasticity at the individual level are completely different processes and may or may not lead to equivalent predictions regarding trait covariation.”
The same argument applies to trait covariation. If environments that tend to favour large bodies also tend to favour high parental investment, then genes controlling the two traits will come to covary across species. There is no equivalent evolutionary process creating trait covariation across individuals. Selection and evolution can lead to phenotypic plasticity and adaptive calibration of individuals’ traits to their personal environments; but Darwinian phenotype-environment matching at the species level and plasticity at the individual level are completely different processes and may or may not lead to equivalent predictions regarding trait covariation. Similarly, there are different consequences at the species and individual levels in situations where selection on one trait depends on the level of another trait. If genes for high parental investment are advantageous in a large species and disadvantageous in a small species, genes for large body size and genes for high parental investment will tend to go together across species, and likewise genes for small body size and low parental investment. But there is no equivalent evolutionary process by which variation in different traits within a population can be combined based on how well they work together. Darwinian evolution cannot combine “tall alleles” with “high parental investment alleles”, and “short alleles” with “low parental investment alleles”, within a sexually reproducing population in the same way as it can across species, because of Mendel’s Law of Independent Assortment: segregating alleles are transmitted to offspring independently of each other, so allele configurations are not inherited.
We explain why exceptions to Mendel’s Law of Independent Assortment – correlational selection, physical linkage, non-random mating, and pleiotropy – don’t change our central point. We also discuss adapted developmental plasticity, where variation and covariation of life history-related traits is shaped by environmental conditions during an individual’s development. In particular, we note behavioural genetics research that shows little influence of early environment on trait variation and covariation, and why these findings pose problems for these plasticity-based life history accounts. Further, we discuss why proposed trade-offs between specific life history traits – trade-offs that are typically on the within-individual level – need not translate to covariation of those traits across individuals.
“Much of the life history work on trait covariation in humans does not have a solid theoretical grounding; there is little basis for the fast-slow continuum as a framework for understanding human trait covariation.”
Overall, we conclude that much of the life history work on trait covariation in humans does not have a solid theoretical grounding. In particular, there is little basis for the fast-slow continuum as a framework for understanding human trait covariation. Given the frequent attacks on evolutionary psychology from other disciplines, our field must be especially vigilant for conceptual flaws in the underpinnings of our research – otherwise, we risk validating those criticisms. Those using life history theory to study individual differences and trait correlations must ensure their paradigm is properly connected to the fundamentals of evolutionary theory.
Read the paper: A critique of life history approaches to human trait covariation