According to the insurance hypothesis proposed by Nettle et al., “humans possess evolved mechanisms that respond to cues or experiences indicating that access to sufficient food is uncertain by increasing energy intake relative to expenditure, and hence storing more fat” (sect. 6, para. 1). However, given the current “obesogenic” environment of highly available energy-rich food, it is unclear why modern humans, at least in the Western world, might experience food insecurity. One possible explanation is that some individuals have a reduced ability to imagine and plan for the future, and that this may lead to feelings of insecurity regarding the future availability of food.

The ability to imagine and plan for the future is known as “episodic foresight” and is one half of an overarching ability to re-experience the personal past and imagine the personal future, known as “mental time travel” (Tulving Reference Tulving1993). There is now a general consensus that ability in episodic memory and episodic foresight are linked within the individual. They involve common mental processes (Schacter & Addis Reference Schacter and Addis2007), engage the same neural substrates (Buckner & Carroll Reference Buckner and Carroll2007), and are impaired in the same patients (Hassabis et al. Reference Hassabis, Kumaran, Vann and Maguire2007; Tulving Reference Tulving1985). They also develop at the same time in children (Suddendorf & Busby Reference Suddendorf and Busby2003; Reference Suddendorf and Busby2005) and decline in the same way in the elderly (Addis et al. Reference Addis, Wong and Schacter2008). These abilities appear also to be linked evolutionarily. Animals that exhibit behaviour suggestive of episodic memory have also been found to plan for the future (Cheke & Clayton Reference Cheke and Clayton2012; Clayton & Dickinson Reference Clayton and Dickinson1998; Correia et al. Reference Correia, Dickinson and Clayton2007; Raby et al. Reference Raby, Alexis, Dickinson and Clayton2007).

Growing evidence also suggests that mental time travel ability influences and is influenced by obesity. The decision to eat a given food is controlled not only by homeostatic signals, but also influenced by higher neural systems – for example, the hippocampus (Berthoud Reference Berthoud2011). The hippocampus is an integral brain region for mental time travel (Schacter et al. Reference Schacter, Addis and Buckner2008; Simons & Spiers Reference Simons and Spiers2003), and this ability has been demonstrated to play a key role in consumption regulation (Higgs Reference Higgs2002; Higgs et al. Reference Higgs, Williamson and Attwood2008). Patients with bilateral hippocampal damage producing episodic amnesia (a condition that produces severe deficits in both episodic memory and foresight) appear to consume several consecutive meals without reporting satiety (Hebben et al. Reference Hebben, Corkin, Eichenbaum and Shedlack1985; Rozin et al. Reference Rozin, Dow, Moscovitch and Rajaram1998). Furthermore, in rats, selective lesions to the hippocampus produce increased food intake and weight gain relative to intact and sham-operated controls (Davidson et al. Reference Davidson, Chan, Jarrard, Kanoski, Clegg and Benoit2009).

Research in rodents has shown that both diet-induced and genetic models of obesity are associated with impaired memory function and hippocampal damage. For example, rats maintained on a high-fat, high-sugar diet display spatial memory deficits after only 72 hours, and non-spatial memory deficits after 60 days (Kanoski & Davidson Reference Kanoski and Davidson2010). In addition, rats fed a sucrose or fructose solution for four weeks showed a 40% reduction in hippocampal neurogenesis and increased hippocampal apoptosis compared to the water solution control group (van der Borght et al. Reference van der Borght, Kohnke, Goransson, Deierborg, Brundin, Erlanson-Albertsson and Lindqvist2011). Genetic models of obesity demonstrate similar memory deficits (Li et al. Reference Li, Aou, Oomura, Hori, Fukunaga and Hori2002; Winocur et al. Reference Winocur, Greenwood, Piroli, Grillo, Reznikov, Reagan and McEwen2005) often accompanied by hippocampal dysfunction (Li et al. Reference Li, Aou, Oomura, Hori, Fukunaga and Hori2002). Hippocampal dysfunction has also been shown in overweight humans (Jagust et al. Reference Jagust, Harvey, Mungas and Haan2005; Raji et al. Reference Raji, Ho, Parikshak, Becker, Lopez, Kuller, Hua, Leow, Toga and Thompson2010), and there is evidence to suggest this is associated with episodic memory deficits (Cheke et al. Reference Cheke, Simons and Clayton2016; Cournot et al. Reference Cournot, Marquie, Ansiau, Martinaud, Fonds, Ferrieres and Ruidavets2006; Gunstad et al. Reference Gunstad, Paul, Cohen, Tate and Gordon2006a). Although there is, to date, little research investigating episodic foresight in obesity, overweight/obese individuals have been shown to have problems with planning tasks, including the Tower of London task (Gunstad et al. Reference Gunstad, Paul, Cohen, Tate, Spitznagel and Gordon2007; Sweat et al. Reference Sweat, Starr, Bruehl, Arentoft, Tirsi, Javier and Convit2008).

This evidence suggests that obesity is associated with neural and psychological deficits consistent with impaired mental time travel. Such a reduction in the ability to learn from personal experience, and to extrapolate that experience to imagine the personal future, may lead to feelings of uncertainty and insecurity about the environment, and the availability of resources. As such, it may be that, although high-energy food is more available than ever before, psychological uncertainty may give the impression of uncertain resources, which the insurance hypothesis suggests may lead to increased energy storage and consequent body fat.

In summary, we suggest that impaired hippocampal function and mental time travel may be a key mechanism underpinning the insurance hypothesis, creating the perception of limited resources even in a plentiful environment. Furthermore, given evidence that deficits in mental time travel can both precede and follow the development of obesity, it may be that this forms part of a vicious cycle in which memory and planning deficits promote obesity, which itself reduces the ability to remember and plan.