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1. Introduction
The prevalence of obesity and overweight is increasing across almost all countries of the world (NCD Risk Factor Collaboration 2016; Wang et al. 2011). This is considered to constitute a major global public health challenge. Despite the societal importance of the topic, there is a dearth of well-developed explanatory theories for why some people become obese. Weight gain occurs when individuals habitually consume more energy than they use. Thus, decision making – in particular, decision making about how much and which foods to eat – is a central and necessary node on the causal pathway to weight gain. Decision making must, in turn, be underlain by decision-making mechanisms. It is the operating principles of these mechanisms that we need to understand: Under what circumstances will individuals recurrently make decisions that lead to their habitual consumption of more calories than they immediately require?
In this article, we advance and review one particular hypothesis concerning obesity. We will call this the insurance hypothesis (IH). We will lay out the hypothesis and its predictions over the course of the article, but it is worth stating its main constituent claims up-front:
- Storage of body fat is an adaptive strategy used by many vertebrates, including humans, to buffer themselves against periods during which food is unavailable.
- Fat storage also has costs.
- The optimal level of body fat to store, therefore, depends on security of access to food: If food is guaranteed to be always available, relatively little fat storage is necessary, but as the risk of temporary unavailability of food increases, the amount of fat the individual should optimally store also increases.
- Humans and other vertebrates possess decision-making mechanisms that adaptively regulate their fat storage. These mechanisms cause them to increase their energy intake above their level of energy expenditure when they receive cues from their environment that access to food is insecure, and reduce their energy intake to close to their expenditure when they receive cues that access to food is secure.
- A major driver of obesity and overweight among contemporary humans is exposure to cues that, over evolutionary time, would have reliably indicated that access to food was insecure. Exposure to these cues engages evolved decision-making mechanisms and leads to increased food consumption relative to expenditure,...