Is being overweight just a lack of willpower?

Twin studies give the clearest picture of how much biology weighs. A systematic review of 88 estimates (Elks et al., 2012) found that the heritability of BMI ranged from 47 to 90 percent, with a median near 75 percent. That is a population-level estimate: it describes how much of the variation between people is due to genes, not how far your own weight is predetermined. Genes explain a large share of why two people in the same environment settle at different weights.

The body also defends its weight. When 50 people in one study (Sumithran et al., 2011) had lost an average of 13.5 kilograms, their appetite hormones shifted: satiety signals fell, hunger signals rose, and subjective hunger stayed elevated a year later. The loss itself makes the body more inclined to regain, regardless of how disciplined the person is.

A review in Nature Reviews Genetics (Loos & Yeo, 2022) describes the mechanism: the brain's leptin-melanocortin system regulates hunger and energy balance, and the same environment produces different outcomes depending on inheritance. Willpower is part of the picture, but it works inside these limits, not above them. Behaviour matters, yet the resistance it works against differs from body to body.

The myth survives because it is simple and feels fair. 'Eat less, move more' places the whole burden on the individual and fits a deep-seated belief that the body mirrors character, an attribution bias where we explain other people's weight by willpower and our own by circumstance. It also serves sellers: a product or programme can promise a fix only if the problem is framed as a lack of discipline rather than biology you do not control. And it is cheap for everyone except the person carrying the weight, since blame costs nothing while structural change costs a great deal. The result is shame, and shame rarely drives lasting change.

Biology weighing heavily does not mean nothing can be changed. It means expecting resistance and planning for it, rather than reading hunger as a moral failure. Slow, sustainable changes in food and movement work better than strict spells the body later regulates away. And if weight is affecting your health, medical assessment belongs with a clinician, not in a comparison with other people's willpower.