There are clear and unan­i­mous fig­ures that obe­si­ty has increased over time, espe­cial­ly in rich nations like the Unit­ed States. On aver­age peo­ple today weigh more and have high­er body fat per­cent­ages than their recent ances­tors at sim­i­lar ages. But why is that the case? One wide­spread assump­tion is the palata­bil­i­ty the­o­ry. It posits that the pri­ma­ry cause is an increase in reward sig­nals in the mod­ern envi­ron­ment from processed food, medi­at­ed by the moti­va­tion sys­tem in the brain, caus­ing us to overeat.