Bygren wondered what sort of long- term effects these drastic changes had on the people of Överkalix. He picked 94 men to study. Studying church records, he charted their genealogies and discovered a correlation between their own health and the experiences of their grandfathers. Men whose paternal grandfathers lived through a feast season just before puberty died years sooner than the men whose grandfathers had endured a famine at that same point in their life.
Women, Bygren found in a later study, also experienced an influence across the generations. If a woman’s paternal grandmother was born during or just after a famine, she ended up with a greater risk of dying of heart disease. It had long been known that a woman’s health while she was pregnant could influence a fetus, but Bygren’s research suggested the effects could stretch even further, to grandchildren or beyond.
Experiments on animals produced some similar results. In the early 2000s, Michael Skinner, a biologist at Washington State University, and his colleagues stumbled across one while they were investigating a fungus- killing chemical called vinclozolin. When Skinner and his colleagues gave vinclozolin to pregnant rats, their offspring, and even their grandsons, developed deformed sperm and other kinds of sexual abnormalities.
Skinner’s work inspired other researchers to look for other kinds of changes that could be inherited. Brian Dias, a postdoctoral researcher at Emory University, wondered if mice might even pass down memories.
Each day, Dias put young male mice in a chamber into which he periodically pumped a chemical called acetophenone. It has an aroma that reminds some people of almonds, others of cherries. The mice sniffed the acetophenone for 10 seconds, upon which Dias jolted their feet with a mild electric shock.
Five training sessions a day for three days was enough for the mice to associate the almond smell with the shock. When Dias gave the trained mice a whiff of acetophenone, they tended to freeze in their tracks. Dias also found that a whiff of acetophenone made the mice more prone to startle at a loud noise. In other trials, Dias would pump an alcohol- like scent called propanol into the chamber instead, without giving the mice a shock. They didn’t learn to fear that odor.
Ten days after the training ended, researchers from Emory’s animal resources department paid Dias a visit. They collected sperm from the trained mice and headed off to their own lab. There they injected the sperm into mouse eggs, which they then implanted into females.
Later, after the pups had matured, Dias gave them a behavioral exam, too. Like their fathers, the new generation of mice was sensitive to acetophenone. Smelling it made them more likely to get startled by a loud sound, even though he had not trained the mice to make that association. When Dias allowed this new generation of mice to mate, the grandchildren of the original frightened males also turned out to be sensitive to acetophenone.