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What part do truth and evidence play in our beliefs? A larger part in ours than in those of others, of course, for we are, or at least consider ourselves to be, more rational almost by definition than anyone else. But if we are honest, which we so rarely are, we must admit that we generally fall far short of Bertrand Russell’s notion of the rational man, which is to say he who holds his beliefs in exact proportion to the evidence in their favor. I have never met anyone like that, and Russell certainly wasn”€™t like that himself.

I suppose the nearest I ever came to changing my beliefs in pure accordance with the evidence is when I read the papers in The Lancet that proved that peptic ulceration was caused in the majority of cases by infection with the bacterium Helicobacter pylori. Until then, all sorts of theories of causation had been proposed, generally a mishmash of dietary, physiological, and psychological factors. Helicobacter swept them all away, like last year’s best-sellers, and I was converted without an inner struggle.

On reflection, the ease with which I was converted was attributable to the fact that I had previously held no other theory of causation of peptic ulceration of my own, strongly or even weakly. I had no belief at all to defend, and therefore was unusually open to the evidence. The mind abhors a vacuum, and Helicobacter encountered no difficulty in filling it.

This is not a typical situation, however. For example, I have just read a paper in The New England Journal of Medicine about a proposed genetic mechanism of obesity. Irrespective of the evidence presented, I admit that I was viscerally ill-disposed to an explanation that almost certainly will be used to absolve mankind from its own folly, from its increasing tendency to blubber, which is particularly grotesque in the Anglo-Saxon world but is spreading at an alarming rate elsewhere.

The reason I was hostile even before I had read the paper fully was its threat to my deeply held belief that obesity is not straightforwardly a disease like any other, but rather (in most cases) the consequence of human weakness. I hold this view not only or even mainly because of the evidence in its favor, but because I am afraid that to hold the opposite view, that obesity is in principle no different from, say, Parkinson’s disease, is to turn mankind from subject to object.

Strictly speaking, of course, to hold that some people are genetically predisposed to becoming fat, on the one hand, and that obesity is the consequence of human weakness, on the other, is not contradictory: But which of these two propositions you emphasize reveals your philosophy of life. Anxious to preserve my worldview, I read the paper (much of whose science I did not really grasp) with a view to sniffing out error, as an Inquisitor sniffed out heresy. And, not surprisingly, I soon found what I was looking for.

In the introduction to the paper, the authors write:

Body-mass index (BMI) has a strong genetic component (40 to 80% heritability) involving several genes that have expression in the hypothalamus and fulfill roles in appetite regulation.

The body mass index is calculated by the person’s weight in kilograms divided by the square of his height in meters, and (roughly speaking) the higher it is, the fatter he is.

Most people would conclude from this”€”and I suspect that they are supposed by the authors to do so”€”that a person’s body size is roughly 50 percent dependent upon his genetic inheritance, but this is false. Genetic variance may explain a proportion of variance within populations but not necessarily between them (and even the degree to which a trait such as obesity is dependent on genetic variation itself may be dependent on nongenetic conditions). If a population as a whole gets fatter while its genetic endowment remains the same, the cause of its newfound obesity is not genetic, even if it has a genetic predisposition to obesity.

The paper thus holds out the age-old false hope that we can become good, sensible, or (in this case) temperate by purely technical means that require nothing of us as moral beings endowed with agency except compliance with treatment and obedience to technicians. Appetite itself will come under the control of geneticists, who will relieve us of the necessity to exercise self-control.