When birds of a feather flock separately

Identical twin studies have been a favorite tool in the nature-versus-nurture debate for almost a century. If two people share 100 percent of their DNA but live different lives, the differences between them are presumably environmental. The classical twin studies built a lot of our early thinking about heritability. The newer twin studies are doing something different. They are showing how much of identical-twin variation comes from differences in the prenatal environment, before either twin has had a single day of independent experience.

What the recent work shows

Several research groups have looked at brain development in identical twins where one twin developed a neurodevelopmental condition, autism or schizophrenia in particular, and the other did not. The differences trace back surprisingly far. Differences in placental blood flow. Differences in nutrient delivery. Differences in maternal hormone exposure depending on placental position. Differences in birth weight. The healthier twin, in most of these comparisons, had a slightly better intrauterine environment.

This is not a comfortable finding for our cultural narrative of identical-twin sameness. It is also, in a different sense, hopeful. It tells us that the prenatal environment is a significant lever, and the pediatric and obstetric community is paying more attention to it than they did a generation ago.

Three points worth noting

1. Identical twins do not share an identical womb. Even when they share a placenta, the blood flow through that placenta is rarely perfectly equal. Calorie delivery, oxygen delivery, hormonal exposure, and toxin exposure can all differ between twins by enough to produce different developmental outcomes.
2. Prenatal effects can have long latency. Some of the most striking findings, like the association between maternal omega-3 intake during pregnancy and later cognitive performance in school-age children, do not show up for a decade or more. Maternal nutrition during pregnancy is one of the higher-leverage interventions in all of medicine.
3. Some prenatal patterns are not reversible. The brain develops in critical windows. Patterns set during those windows are stable in ways that adult interventions cannot fully undo. The good news is that milder patterns can be substantially improved with the right work.

Why I am bringing this up

I had a friendly argument recently with a friend, also a physician, whose father died at 46 of a specific coronary pattern. My friend, in his early 40s, started having chest pain and was diagnosed with the same pattern. His cardiologist told him this was likely heritable, and the conversation more or less stopped there.

What was not discussed. My friend lived under his father’s roof until age 25. He ate the same diet. He had the same family stress patterns. He went into a profession with chronic sleep deprivation. He has never had his omega-3 index tested, that I know of. He may well have a genetic predisposition. He almost certainly has a long-running set of environmental and lifestyle exposures that are pushing whatever genetic loading he carries into actual disease.

The takeaway for most readers

For the vast majority of people, the question is not whether you are stuck with your parents’ fate. You are not. The question is whether the environment you are running, today, is the one that turns your inherited risk down or turns it up. The epigenetic literature is clear that lifestyle inputs account for the majority of variance in middle-age disease risk. The exact percentage is debatable. The direction is not.

Most of us were dealt a workable hand. The question is how we play it.

— Doc