So identical twins should agree on moral issues twice as often as fraternal twins, at least if genetics has any influence on human compassion.
And, he says, that's precisely what he found.
The participants used a scale of 1 (strongly disagree) to 5 (strongly agree) to evaluate a number of statements, including "Cheating on income tax is as bad as stealing" and "I am a person people can count on."
The answers among identical twins were almost twice as alike as among the fraternal twins, leading Ruston to conclude that genes account for 42 percent of the individual differences in attitudes.
Not everybody will buy that, of course. Paul R. Ehrlich of Stanford University, a leading proponent of the "environment is everything" school of thought, maintains that identical twins have very different life experiences than the rest of us. Even parents sometimes have trouble telling them apart, so their environmental experiences may be more similar than different.
"Genetic evolution did not determine most of how we act or provide us all with a pre-programmed 'human nature,' " Ehrlich told a meeting of the American Institute of Biological Sciences. "There is no reason to believe that human beings are either innately violent or innately peaceful, instinctively disposed to wreck their environments or to be conservationists, or born genetically gay or genetically straight."
"That's silly," Ruston says. The vast majority of scientists in the field believe genes do play a role, he adds.
"It's both," he says. "The genes tell us that some of us are a tad more socially responsive than are others."
That's likely true, even on a gender level, he adds. One of the questions he asked the participants was whether they liked seeing others open presents. A hardy yes means the person is empathetic.
"Females are much higher on that," Ruston says. "In fact, I didn't find a single female in my sample who didn't endorse that item. But 20 percent of males will say 'nope,' they're not interested in watching people open presents."
Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.