Evolutionary theory posits that every psychological, behavioural and physical trait expressed by an organism was selected for by aiding the organism in its survival and/or reproduction, and therefore had a functional purpose for existing at some point in evolutionary time. A trait could never exist if the environment was not conducive for its expression (i.e. hinders the organism in survival or reproduction).
By this logic, it might therefore be reasonably assumed (however inconceivably) that there could have been some adaptive function for the existence of anorexia and bulimia at some point in our evolutionary history. They are annoyances in our modern world because there could be a mismatch between its usefulness in the ancestral past and the environment today.
There are at least five major evolutionary explanations for restrictive eating disorders, each with its own flaws, as outlined by Dr Igor Kardum and colleagues from the University of Rijeka in a 2008 paper reviewing evolutionary accounts of anorexia and bulimia, namely (1) the reproduction suppression hypothesis (Wasser & Barash, 1983), (2) the model of parental manipulation (Voland & Voland, 1989), (3) the sexual competition hypothesis (Abed, 1998), (4) the adapted to flee from famine hypothesis (Guisinger, 2003), and (5) the combined concepts of 'social attention holding power' and the 'need to belong' (Gatward, 2007).
The most palatable (and, thus, conventional and popular) idea tends to revolve around competition among females catalysed by the media (sexual competition hypothesis). It is the notion that in wealthier societies, to be able to resist food is seen as a mark of having high status, while in poorer societies, to be able to get food is conversely the indicator of high status; thus anorexia is more prevalent in modernised societies while relatively larger-sized females are exemplified as beautiful. This is propagated via media imagery.
I will not go into detail with all the ideas as it can get rather technical, but in particular I wanted to point out two radical ideas put forth that, while still works-in-progress, were really interesting and reflective of creative, convention-defying attempts to think of new ways to consider this issue.
In particular, I thought Voland and Voland's (1989) model of parental manipulation interestingly attempts to account for why eating disorders tended to happen more with wealthier, higher class individuals. This model draws on kin selection theory and asserts that anorexia may be adaptive (useful) insofar as it increases an anorexic's helping behaviour to her own kin's survival and reproduction while suppressing her own reproductive success (because anorexia leads to a decline in fertility). Non-evolutionary research in the 70s showed that members of anorexic families possessed mutually overprotective attitudes, and anorexic individuals tended to worry constantly about the well-being of their families. In the ancestral past when families were larger, anorexic female helpers could suppress their own reproduction and therefore divert their own resources towards helping collateral kin, leading to greater inclusive fitness (genes belonging to family members). This sets the precedent for some interesting speculations. The model of parental manipulation suggests that anorexia is actually somehow exacerbated by parental influence. Many studies report significant correlations between dominant and overprotective mothers and the probability of anorexic reactions of their daughters. Anorexia reduces a female's fertility and hinders her from bearing additional offspring. Additionally, when a daughter is overprotected and dominated by her mother, her ability to find a mate is also reduced. In wealthy families, males are the more valuable sex as they have the resources to attain more mates. By inducing anorexia (thereby restricting the reproduction of daughters and also reducing their food intake), especially for families in higher societal strata and class, parents can then concentrate investment potential towards sons, who in wealthier families have higher reproductive value.
The next radical idea is Gatward's (2007) combined concepts of 'social attention holding power' and 'the need to belong'. 'Social attention holding power' is defined as an individual's ability to hold attention and gain investment from other members of the group, and this concept is closely related to the degree to which a person feels in control. Naturally, higher status individuals hold more social attention in their group and feel more in control. The need for belonging to a group is a fundamental human need, as ancestors who did not belong to any group were unlikely to survive for long in the harsher environments of the past. Because survival depended on belonging to a group, people had to compete for resources and this competition could lead to exclusion, if one wasn't careful. Anorexia might therefore have been adaptive in the past to prevent competition for food and resources, as well as compete in a more nuanced manner for status (a reference to the more conventional sexual competition hypothesis outlined above), thereby promoting group harmony and reducing the likelihood that one might get expelled. I would personally go on to speculate that in our modern society (essentially functioning and subconsciously perceived as a really large group), nobody feels like the highest status female who doesn't need to conform to restrictive eating disorder, because there is an implicit assumption that the highest status female is the one they see artificially created by the media. So everyone else who feels subordinate will increase her tendency to engage in restrictive eating disorder, an adaptation brought on by the need to maintain the large group's social harmony.
These are certainly radical theories that need to withstand more empirical testing, but it is exciting to read them because they represent interesting attempts to get away from more conventional and acceptable ideas that do not necessarily get us very far, as evidenced by the fact that many extant theories on the causes of eating disorders still have ambiguities and gaps. Of course, there has to be a deal of initial plausibility, lacking which we would just think the idea is quack. But science is ultimately pushed by great thinkers with ground-breaking insights.
On a concluding note, the authors of the review paper highlight one consistent element found among all proposed evolutionary theories of restrictive eating disorders - response to threat. All the major evolutionary explanations can be reasoned as a form of response to threat (to survival and/or reproduction) that leads people (especially females) to develop symptoms of eating disorders. This independently corroborates research linking eating disorders with feelings of insecurity and need for control.
Abed, R. (1998). The sexual competition hypothesis for eating disorders British Journal of Medical Psychology, 71 (4), 525-547 DOI: 10.1111/j.2044-8341.1998.tb01007.x
Gatward N (2007). Anorexia nervosa: an evolutionary puzzle. European eating disorders review : the journal of the Eating Disorders Association, 15 (1), 1-12 PMID: 17676667
Guisinger S (2003). Adapted to flee famine: adding an evolutionary perspective on anorexia nervosa. Psychological review, 110 (4), 745-61 PMID: 14599241
Kardum, I., Gračanin, A., & Hudek-Knežević, J. (2008). Evolutionary explanations of eating disorders Psychological Topics, 17 (2), 247-263.
Voland, E., & Voland, R. (1989). Evolutionary biology and psychiatry: The case of anorexia nervosa Ethology and Sociobiology, 10, 223-240
Wasser, S., & Barash, D. (1983). Reproductive Suppression Among Female Mammals: Implications for Biomedicine and Sexual Selection Theory The Quarterly Review of Biology, 58 (4) DOI: 10.1086/413545