I have witnessed a lot of obesity bias in my career, and I have railed against it on more than one occasion. But it’s not the focus of my career, as it is for some of my colleagues. I have long thought those colleagues who do devote themselves to the topic may tend to overlook a very fundamental question, namely: Why would obesity bias even exist? What rational basis is there for people — any people — to be biased against other people because of adipose tissue?

To me, this question is important — because while we may not fix problems we do understand, we almost never fix problems we don’t understand. If we are to banish obesity bias, among doctors or any other group, we need to know it at its origins. We need to get to bedrock.

In general, the bedrock for explaining human behavior is evolutionary biology. Modern culture may refract the imperatives of biology, but they tend to persist, if in new guise. In biology, for instance, food tends to be the currency of survival, and so modern culture has conflated currency and food: We make dough, win bread, and bring home the bacon.

I believe obesity bias has origins in evolutionary biology as well, related directly to the one great imperative of our selfish genes: survival. Survival, in turn, depends greatly on food and sex. And so it is that this is a story about sex and food. Let’s start with sex.

All other things being equal, females tend to prefer strong, well-muscled males. This is true of human females, but extends to most mammals, and other animal classes as well. And it makes perfect sense. In a natural world, strong males are likely to do a better job of securing food, and defending hearth and home. Females, and their genes, would tend to favor males, and male genes, associated with vigor and strength that foster the family unit’s survival, and thus the propagation of the very genes that favored this scenario in the first place.

Of course, modern culture does indeed refract this. In cultural context, wealth and power not mediated by muscle may substitute for the more primitive form of vitality. Men may benefit from this, and the relevant research suggests that obesity bias and its consequences disproportionately encumber women.

Across human cultures, males tend to favor females not of any given size, but with certain proportions. In cultural context, those proportions have been dressed up with all the mystique of sex. But they are really a visual cue about probable reproductive success. Just as females tend to favor males who look like they can put bread (or mammoth) on the table, males tend to favor females who look like they can deliver, and nurture, babies. The famous male fascination with breasts is really a message from survival-driven genes about the likelihood of well-fed offspring.

And since we are very visual creatures, these messages about sex and survival are mediated to us principally by our eyes. If we were more dependent on scent, or sound, we might be less influenced by body shape and size, and more motivated by other factors. But this, too, is part of our biology, and so it is the visual cues that predominate.

So here we have point one: There are almost certainly prevailing preferences for certain standards of physical appearance that are hardwired into us all, because they have long related to reproductive success. This doesn’t make them right, or good — but it does make them real and important. We are not likely to be masters of our behaviors when we ignore the reasons for them.

That’s the sex story; what about food? Here I am less confident, and acknowledge that while I write with professional credentials on the obesity topic, I am only a very amateur anthropologist. I have read a lot of anthropology, but don’t have credentials. I would appreciate the impressions of any real experts who happen upon this.

Body fat is a form of energy storage. The safest, most reliable place to put a surplus of calories today against the advent of a rainy day tomorrow is inside your own skin, so you can take it with you wherever you go. In the context of evolutionary biology, the reason our bodies can and do grow adipose tissue is because the capacity to store excess food reserves today to weather tomorrow’s famine favored survival.

In a world where calories are a rate-limiting commodity in the struggle to survive, as they were throughout most of human history, it might be that you and I are to some extent competing to get enough. And if you manage to grow adipose tissue, it might increase the risks of me not getting enough calories to survive. Maybe obesity evokes some antipathy because it threatens us. Maybe in native context it hints at someone getting more than their fair share, and putting the rest of us at risk of starving. This is just conjecture on my part, admittedly — but it certainly makes sense.

And then lastly, in terms of basic biological impulses, there is our innate xenophobia. Xenophobia is a pretty deplorable tendency, as it accounts for many of the horrible abuses human beings have perpetrated on other human beings who looked different. We have done this because of skin pigment. We have done this because of facial features. It redounds to our great shame, but it, too, makes sense in basic biological context.

Throughout the long sweep of human prehistory, we lived in small and widely-dispersed groups. Members of our own group worked with us and fostered our survival. Members of any other group might try to take from us, and profit at our expense. Familiarity meant safety; foreign meant dangerous. And since obesity has been rare throughout most of human history, maybe it still evokes the foreign, and taps into some subconscious aversion, borne of fear.

These are potential sources of obesity bias in the population at large. What of doctors specifically?

Well for one thing, doctors are just people, and so are prone to all the same foibles as everyone else. But I think these are compounded by frustrated impatience, and lack of self-efficacy. Doctors, after all, are trained to fix what’s broken. When hearts stop, we restart them. When wounds bleed, we suture them closed. When bones break, we set them. When tumors grow, we cut them out.

But we are not very good at fixing obesity, which is of course far more a cultural than a clinical problem. Not being able to fix what you are supposed to fix does not tend to bring out the best in people. In the case of doctors, this is by no means limited to obesity. Anyone with a “syndrome” has likely encountered this same brand of frustrated intolerance. In medicine, if we understand and truly respect a condition, it gets promoted to a “disease.” If we don’t understand, and aren’t entirely committed to respecting it, it languishes as a syndrome. In my 20 years of clinical experience, patients with syndromes, and patients with obesity, have reported pretty comparably unpleasant experiences.

So much, then, for the nature of the problem. What of the solutions?

For doctors, it begins in the training process. We can, and should, make it clear in every variation on the theme of medical education that obesity bias is wrong, unjustified, and impermissible. We need doctors and doctors-to-be, and all other health professionals for that matter, to understand the culture-wide origins of epidemic obesity, and the limits of personal control over it. We also need to train health professionals to help patients manage their weight so there is a better option than frustrated intolerance.

And here, I am not just opining; I have put years of effort where these words are. I am currently writing the third edition of a nutrition textbook for health professionals with dedicated chapters on weight management and counseling. And my colleagues and I have developed, and offer free of charge, an online training program in constructive, compassionate lifestyle and weight-control counseling that carries with it four hours of category one continuing medical education credit from Yale University, at no cost. To my colleagues reading this: Please help yourselves!

But the best means of fixing obesity bias among doctors is fixing it everywhere, banishing it from our culture. This bias is wrong, unfounded, and counterproductive. Our society has tolerated it openly, and sanctioned it tacitly, for too long already.

If my arguments above are sound, then fixing obesity bias requires replacing mindless imperatives coded into our genes with thoughtful products of our culture. Richard Dawkins, arguably the father of modern thinking in evolutionary biology, makes the case for just this kind of substitution in his groundbreaking book The Selfish Gene. Genes are bequeathed to us by biology, blind to all but survival. But memes, units of cultural transmission, are of our devising. We can decide which matter, and propagate them accordingly.

Obesity bias needs to be fixed. We are most likely to fix it when standing on a solid bedrock of understanding it at its origins. We can then replace the crude and obsolete survival-related imperatives bestowed to us in our genes, with the better angels of our nature, borne of culture, mediated by memes. If not us, who; and if not today, when better?

David L. Katz is the founding director, Yale-Griffin Prevention Research Center.