If you have a genetic inclination to diabetes, you are not alone.  According to one estimate, there are 171 million people in the world with diabetes, and the number isn’t going down.  Is there anything good about having a gene that predisposes a person toward diabetes, or high cholesterol, or Alzheimer’s disease? 

And what about favism?  Favism is the most common enzyme deficiency in the world, which can in extreme cases lead you to die from merely eating a plate of fava beans.  Yet the genetic inclination toward favism is, oddly enough, most common throughout the Mediterranean region, where fava beans are a staple food.   

In their book Survival of the Sickest, evolutionary physiologist Sharon Moalem teams up with journalist Jonathan Price to address the question of how genetic diseases like diabetes or favism could possibly evolve, given their very high costs.  The answer has potentially profound implications for understanding psychological disorders, such as schizophrenia, depression, and anxiety disorders.  It’s an answer that is becoming increasingly familiar to anyone who thinks about the evolution of human behavior, and it can be expressed very simply: Trade-offs.  

Trade-offs: You find them everywhere. 

All evolved traits come with benefits.  If they didn’t, they wouldn’t evolve.  But just as there is no free lunch, there are no free genetic goodies.  As Moalem and Prince note, when a disease is found in people from a particular population, “it’s time to raise the evolutionary eyebrows and start asking questions – because that almost certainly means that some aspect of the trait that causes the disease today helped the forebears of that population group to survive somewhere back up the evolutionary line.”

The authors discuss evidence that diabetes may be a byproduct of a genetic proclivity that helped Northern Europeans' ancestors survive the sudden cold of the most recent ice age.  Along the way, they discuss tree frogs whose cells are able to survive freezing temperatures during their winter hibernation.  The frogs survive using some of the same metabolic tricks that happen in the bodies of diabetics.  They also discuss evidence that favism is a byproduct of malaria resistance, and that Alzheimers could result from a genetic trait that helped some of our ancestors survive the Bubonic Plague. 

The book is also full of other fascinating facts: about rare (and sometimes scary) diseases (like Guinea worms, which grow up to 3 feet long inside your body, then burn their way out using a painful acid secretion), and about the coevolution of the human genome with the bacteria that live inside our bodies, as well as those that come for occasional (and often deadly) visits.  

This is one of those books that really made me feel like I was learning something new every few pages, it is also one of the minority of nonfiction books that kept me turning the pages, and left me wishing it were a little longer when I got to the end.  It lived up to the promise in the introduction that it would be a “magical medical mystery tour,” with side-trips exploring the evolution of the human brain, the theory that our ancestors were semi-aquatic, the evolution of aging, and evidence that a mother’s experiences can alter the expression of her offspring’s genes. The authors managed to keep the book interesting without doing a journalistic snowjob on the science: They note at many points that a lot of what they are discussing is speculation, and they acknowledge that attempts to manipulate the genome for medical benefit can result in disaster, since it's virtually impossible to go in and simply change one thing in such a complex system.

Adaptations, byproducts, and misfits in the modern world

I opened with the question “Is there anything good about having a genetic inclination toward diabetes or high cholesterol or Alzheimer’s disease?”  The simple answer is that most common heritable diseases seem to be related to genetic proclivities that once had some benefit.  But this is not to say that the disease itself is “good.”  Some of the painful costs of disease symptoms are simply necessary evils, like the damage to the host’s body done by a fever whose main purpose is to kill bacteria.  Some of them are accidental byproducts, like the occasional death linked to the anti-malarial genes predisposing favism or sickle-cell anemia.  And some are the results of mismatches between genes and modern environments.  Northern Europeans' light skin evolved to allow in sufficient Vitamin D in a cloudy world with a winterful of short days, but when that light skin meets the sun doses of Arizona or Southern California, it results in an increase in skin cancer.  On the other side of the skin-color spectrum, the genes predisposing dark skin have benefits in sunny climes, but results in Vitamin D deficiencies, and various health disorders, when transplanted to places like Boston or Seattle. 

Although Sharon and Prince don’t get into it, these same issues apply directly to psychopathology: Many psychological disorders are linked to genes; there are trade-offs that make those genes desirable under certain circumstances (a little anxiety is a good thing, for example); the problems may simply be costs of doing business, or they may be the result of mismatches between ancestral conditions and the modern world.  Jon Maner and I talked about how these issues apply to anxiety disorders, in an article titled "When adaptations go awry."  This way of thinking raises a whole host of issues for how we think about our own problem thoughts, feelings, and behaviors.  That is, of course, a whole ‘nother story.  For now, let me end with a strong recommendation for The Survival of the Sickest.  Although it won’t make you want to die, there’s a good chance it will lead you think differently about whatever natural cause it is likely to kill you.

Douglas Kenrick is author of Sex, Murder, and the Meaning of Life: A psychologist investigates how evolution, cognition, and complexity, are revolutionizing our view of human nature.  


Maner, J. K., & Kenrick, D. T. (2010). When adaptations go awry: Functional and dysfunctional aspects of social anxiety. Social Issues and Policy Review, 4, 111-142

Moalem, S. & Prince, J. (2007).  Survival of the sickest.  New York: HarperCollins. 

About the Author

Douglas Kenrick

Douglas T. Kenrick, Ph.D., is professor of social psychology at Arizona State University.

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