Studies suggest that some changes to the epigenome may be passed on to our future grandchildren. Meanwhile, scientists are actively working on epigenetic editing—finding a way to hack gene expression. Others are developing drug treatments that target the epigenome. There is hope that the science of epigenetics will one day help doctors detect and disrupt diseases earlier and more effectively.

Whether a gene is active or not can depend on chemical compounds that click onto the DNA structure, toggling the gene’s on-off switch (think of it as a biological lock and key). These changes can be clicked into place, and they can also be undone. Environment and lifestyle can influence gene activity, and it is within this segment of the field of epigenetics that twin studies can play a key role.

My own identical twin boys are now 16 months old. One has a strawberry-shaped birthmark on his left ankle. The other has a thick em-dash-shaped birthmark on the back of his right thigh. One has a hair whorl that swoops to the right. The other’s swoops left. Without those defining markers—for the first six months of their lives especially—my husband and I might have mixed them up and never straightened out the mistake.

It is not as hard to tell my sons apart now, but we often recognize them more based on personality differences than looks. One is adventurous, daring—the first to nosedive off a sofa, the first to fall down stairs. He also crawled, stood, cruised, and walked first. He hollers and cries when we leave the room. Our other boy is an observer. He can be laser-focused, able to spend 30 minutes trying to click together a buckle as his brother marches around with his chest puffed, in need of constant movement and entertainment.

In life, they’ve shared bottles, diets, sleep schedules, and common colds. In the womb, they shared a placenta (but not umbilical cords). Still, they were starkly different long before most of these experiences, from the instant the doctor sliced me open and yanked them out. One screamed all night long in those first days on Earth. The other, in the same bassinet, dozed right through his brother’s cries.

Erika Hayasaki

I wondered: What the heck then was going on inside my body that may have influenced their different personalities in their first shared year of life? No doctor or scientist can possibly tell us for sure. But the epigenetic changes that can take two identical strands of DNA and turn them into two unique individuals are thought to start in the womb. Some studies of twin newborns have shown that intrauterine and postnatal environments lead to differences in gene expression, and some of these divergent patterns are detectable at birth.

These differences may widen as twins grow up. While infant identical twins can be almost indistinguishable, some can begin to look more unique as they age (though friends and family still confuse adult twins all the time, Segal says, as with the Colombian sets). But Segal studied a pair in which one twin was always heavier growing up. “Mom said he always ate more—so there is a subset of twins like that. And adverse prenatal factors can intervene, making identical twins somewhat different in height—the average difference is two inches,” she says. For some pairs, their different environments change them. One may spend more time in the sun. One may smoke or experience greater stress. All of these things could influence their epigenomes.