Unbending rigor is the mate of death,
And wielding softness the company of life
Unbending soldiers get no victories;
The stiffest tree is readiest for the axe.
(Tao Te Ching: 76)

Early in life, our bodies are like unmolded clay, ready to be shaped by our experiences. For some of us, that matching process can create problems. If circumstances change, we could end up poorly adapted to our adult environment. A child born into harsh conditions, though, may have to take that risk in order to make it to adulthood at all.

The Hmong in French Guiana may be an example of this process. They are a fascinating population for many reasons, the most obvious being that they are there at all. A few dozen refugees from Laos first resettled in French Guiana in 1977, a few years after the Vietnam War, after they and the French government agreed that life in small, ethnically homogenous villages in a tropical environment was a better option than acculturating to the cities of Métropole France. The experiment paid off. Today, more than two thousand Hmong are farmers in the Amazonian jungle, producing most of the fruits and vegetables in the country. The result is a level of economic autonomy and cultural retention that is likely unique in the Southeast Asian refugee diaspora.

Scenes from Hmong villages in French Guiana. (Clockwise from top left: fields of Cacao, young men going on a hunting trip in Javouhey, swidden agriculture of Cacao, a street lined with farmers’ trucks in Javouhey.

They are also interesting from a biological perspective, as an example of how adverse experiences from our pasts can remain subtly embedded in our bodies. As a graduate student, along with a research assistant, I visited the Hmong villages of Cacao and Javouhey in rural French Guiana. We asked people there if we could interview them about their lives, including where they were born, what life was like for them during the war (which engulfed not only Vietnam, but neighboring Laos and Cambodia as well), and how old they were when they moved to French Guiana. We also measured height, weight, and body fat.

Younger Hmong adults, who moved to French Guiana as infants or children, were taller than the older generation born in the mountains of Laos. That wasn’t surprising. Many studies show that the average height of a population can fluctuate between generations as economic and environmental conditions change. People who moved as children to French Guiana, with its economy subsidized by France, spent more of their early lives in good circumstances. By comparison, older adults grew up in one of the poorest countries in Asia in the middle of a war. Those living conditions were reflected in their final adult stature.

But the study’s main purpose was to find whether people’s war experiences might have influenced their growth. Some people had a much harder time during the war years than others. The history of the war in Laos is a remarkable story unto itself, with many factions involved from several nations. Suffice it to say here that – as in nearly all wars – civilians were severely affected, particularly those who resided close to the Vietnam border. Many people told us they experienced hunger as children, had family members who died from illness or violence, or were forced to flee from their homes multiple times because of the war, in some cases walking dozens of miles to safety.

Such experiences, especially when they occurred early in life, were correlated with how their bodies grew and developed. People who were forcibly displaced by war during the vulnerable period of infancy were significantly shorter as adults. Those who were born in a village near a conflict zone had more body fat as adults, particularly around their abdomen (a type of fat that has been linked to more adverse health outcomes). These results were consistent with other studies looking at how war, or adversity in general, can get “under the skin” to influence how our bodies grow and develop.

Biologists and humanitarian aid workers see childhood growth as a rough gauge of environmental conditions, including nutrition, infection, and even psychological stress. War consistently disrupts these variables. We only need to look at recent photographs from Syria to get a sense of how difficult it would be for children to grow under those conditions. To use an economic analogy, biological resources can be seen as a form of currency. When nutrients are scarce, are spent fighting infection, or are lost through diarrheal disease, a child cannot invest those nutrients in growth.

Why should bodies respond to adverse conditions the way they do? James Tanner, considered to be the founder of modern growth studies, once wrote that “what is inherited is DNA. Everything else is developed.” Through that process of development, multiple biological outcomes from the same genes are possible.

Evolutionarily speaking, it makes sense that natural selection would favor genes that give an organism some plasticity to respond to environments that change within a single lifetime. For example, if an animal’s genome directed it down a rigid, predetermined pathway toward large body size regardless of circumstances, this could backfire if nutritional conditions were poor as the result of an unforeseen drought or flood. Or a war. In those conditions, the metabolic costs of growing and maintaining a large body could outpace available resources, with potentially fatal results. After all, the stiffest tree is readiest for the axe.

By contrast, genes that could somehow “read” the environment and shift the growth trajectory on the fly could prove advantageous. For similar reasons, the zoologist Matt Ridley referred to plasticity as evolution’s “masterstroke” because genes that could “flexibly calibrate” themselves and their protein products could increase an organism’s chances to survive and reproduce in a wider array of environments. And humans can live in a broader range of circumstances than any other animals.

Through the work of David Barker and many others, we now have a good deal of evidence that an adverse prenatal environment can lead to metabolic reprogramming in the human fetus, as well as in other mammals such as rats and sheep. Cambridge biologist Patrick Bateson and colleagues suggested that it is almost as if a pregnant mother living in a harsh environment could somehow give her fetus a “weather forecast” of the outside world, prompting her unborn offspring to forego investing in longevity and large body size. Instead, a better path would be to focus on traits linked to short-term survival, such as small size and reduced muscle mass, a thrifty metabolism that would spare glucose for the most essential organs, a hair-trigger stress response to remain on high alert, and increased body fat storage (a so-called “spare tire”).

However, few things in biology are cost-free. The downside to these growth shifts includes an increased risk for health problems down the road, such as diabetes, cardiovascular disease, and ultimately a reduced lifespan. All things considered, this is a good trade. Getting diabetes in adulthood certainly beats not making it to adulthood in the first place. Because we see these patterns in other species, they likely evolved before humans came on the scene as a way for animals to respond to changes in the local ecology. The difference is that we humans now have the ability to alter our environments radically and rapidly through agriculture (and its failures), public policies, economic recessions, and of course military conflict.

This pattern is what we see in several populations that have gone through enormous hardship. People who were prenatally exposed to the Hunger Winter in the Netherlands during World War II today have an elevated risk for many chronic diseases. In 2005, upon finding that rates of diabetes in Cambodia were much higher than expected, Hillary King of the WHO speculated that this could be yet one more piece of the “grim legacy” of the Killing Fields period and the Khmer Rouge regime. A more recent epidemiologic survey showed that babies born during an intense famine in Nigeria during the Biafran civil war forty years ago are more susceptible to obesity today.

The Hmong are one more example of the underappreciated, lingering costs of war, which can affect us in more subtle ways than death, injury, and post-traumatic stress disorder. These experiences become embedded under our skin for fundamental biological reasons – because   they help children to survive and eventually reproduce, even under harsh conditions. Without these mechanisms, our species might not have gotten here at all.

This essay originally appeared on the website BeingHuman.org, which I was sad to learn is no longer operating.