“Die Young and Live Fast”Share
What Life History Theory can tell us about reducing pathogen stress in the developing world?
Curing and preventing disease can alleviate suffering and increase quality of life in the developing world in the short term, but reducing the burden of infectious diseases promises even greater long term benefits to individuals and society than you might think.
Life History Theory describes psychological and biological changes that prioritize the short term over long term when there are early cues of a harsh and unpredictable environment. Cues of volatility such as parental absence or poor nutrition caused by infectious disease and poverty can change the psychology of a whole society. Removing these cues might have a significant impact on wellbeing.
In evolutionary terms, the goal of every organism is to have as many of its genes in the next generations as possible. Every organism has a limited amount of energy to allocate to activities such as bodily maintenance, growth and mating, each of which can contribute to successful reproduction. There are a number of different strategies that might increase the likelihood of success. You can grow large, or you can get to sexual maturity as quickly as possible. You can put energy into healing injuries, or into finding partners to mate with. You can have few offspring and try to make them as healthy as possible, hoping that they will be more successful as a result, or you can have a large number of offspring and invest little in each one, hoping that at least some will survive. Those strategies that prioritize maturing and producing many offspring quickly, are called "fast life history" strategies.
The success of different strategies is going to depend on a number of factors. One of the most important is how long an organism has left to live, and a good cue of this factor is the mortality of those around you. Researchers have found that unstable and environments which are unstable and may cue a fast life history. In other words, these cues indicate to the organism that it is best to follow a strategy of reproducing sooner rather than later, because they could die at any moment. A fast life history strategy involves earlier sexual maturation, earlier sexual intercourse, greater number and less investment in offspring. This is because in a volatile environment where people die due to random events, success becomes a matter of luck rather than skill, and so it makes sense to have lots of kids rather than invest heavily in just a few 'highly skilled' ones. Furthermore, the time available to reproduce and rear offspring is limited, so you have to hurry. There are other characteristics associated with a faster life history that I'll discuss in more detail later in this post.
It's important to note that changes in life history occur both over generations and an individual's lifetime. For instance, over time a species that is less often eaten by predators may evolve to reach sexual maturity later, or have larger and fewer offspring. Humans have had a long history of living in diverse environments, as well as having an extended developmental period. Thus we 1) have a long period in which cues about the difficulty of the environment can change our reproductive outcomes and 2) we may have evolved to be especially sensitive to these cues. Cues such as early stress, father absence and nutritional scarcity have been associated with faster sexual maturation, earlier sexual debut, greater number of sexual partners, greater number and less investment in offspring in a variety of species including humans.
Here are a few examples of environmental cues and changes that are relevant to reproduction:
-Tasmanian Devils show a 16 fold increase in early sexual maturation when adults die due to an infectious cancer.
- In women, father absence and parental separation predict earlier age of reproduction, earlier onset of sexual activity, earlier age of first period and shorter duration of marriage (controlling for economic factors and mother's behavior).
- Among hunter gatherers and subsistence farmers "age at first period and age at first birth each occur about 1 year earlier for every 10% decline in child survivorship to age 15".
Some studies in the developed world have shown faster life history in those with chronic illness. However, in the developing world fewer studies have been conducted on life history. One notable exception is this study showing that as pathogen stress (defined as presence of 7 different diseases or infections including malaria and leishmaniasis) increased, father involvement (defined as father proximity to infant and father involvement in infancy and early childhood) decreased. In other words, fathers may be putting more time into having more offspring rather than investing in existing offspring when there are more pathogens in the environment. The same study found high levels of pathogen stress were associated with lower maternal care (e.g. mother proximity to sleeping infant) and earlier age at weaning.
Faster life history doesn't only influence reproductive characteristics but also influences personality and social characteristics. Monkeys exposed to variable feeding environments from infancy were less social and more timid than those exposed to more consistent and resource rich environments. Fast life history has also been associated with steep discounting (the prioritization of current over future rewards), greater impulsivity and greater risk taking with regard to resources (see this,this, and this). Faster life history strategy has been associated with greater aggression, rebelliousness, and more insecure attachment style. These latter links are more tenuous but when faced the likelihood of a small time window it's smarter to take risks for immediate gains rather than playing a safer strategy. It's also smarter not to trust others who are likely adopting a similar strategy of taking greater risks.
Thus, reducing pathogen prevalence and disease burden may change cues of environmental harshness and unpredictability such as father presence, maternal investment, and age at weaning. Those developing in environments in which disease burden is reduced may change psychologically such that they reproduce later* and invest more in their children, engage in less risk taking and prioritize future over present rewards. This more patient and cautious approach to life, which will include fewer and better cared for children, obeying the law, and investing more in long term gains including cultivating skills and assets, could reasonably be expected to have an impact on economic productivity and people's incomes. However, it is hard to quantify how much given existing evidence.
In the next blog I'm going to review the parasite stress model , the idea that parasite prevalence influences population characteristics such as proportion of personality types, religiosity, crime and ethnocentrism.
*One wrinkle in this is that women in the developing world may not be reproducing at rates consistent with a fast life history strategy because of inadequate nutrition. Some women in the developing world have been shown to ovulate less and have lower progesterone than women in developed countries.
This article originally appeared at Giving What We Can