Trump, meanwhile, has maintained that he won’t be bothered by the conflicts, telling the Times first, that “the president can’t have a conflict of interest,” then that “the president of the United States is allowed to have whatever conflicts he wants.”

“In theory,” he suggested to the paper’s writers and editors, “I could run my business perfectly and then run the country perfectly.”

The reason critics doubt Trump’s assertion—that he’s somehow above the lure of profit—is that years of research show even small kickbacks can change well-meaning individuals’ behavior. We know this from studies on another group of wealthy, confident individuals who must daily grapple with both moral and economic matters: Doctors.

Studies have repeatedly shown that receiving gifts or money from a pharmaceutical company makes doctors biased toward that company. A comprehensive review in 2010 found that exposure to information from pharmaceutical companies almost always influenced doctors to increase their prescribing of the drugs the companies were promoting, even when those drugs were less appropriate for the patient. In a study published this month in the journal Social Science & Medicine, Yale professor Marissa King found that in states that banned gifts to doctors from pharmaceutical companies, doctors were less likely to prescribe costly new medications that had few advantages over cheaper, generic alternatives.

“Pharmaceutical representatives are likable people, and we like doing things to please people that we like,” says Peter Mansfield, an Australian physician who founded the promotion-awareness group HealthySkepticism.org and co-authored the 2010 review paper.

Partly in response to findings like these, some states and organizations have cracked down on pharmaceutical swag in recent years. Still, doctors aren’t free of conflicts of interest, since drug companies still fund continuing medical education programs and seek out consulting relationships with physicians, according to Genevieve Pham-Kanter, an assistant professor at Drexel University who has researched conflicts of interest.

Even cheap gifts can create a feeling of indebtedness, playing into our human desire to reciprocate. Branded pens or tchotchkes “make the brand name more easily retrievable from memory and cognitively ease the path for doctors to prescribe more of that particular brand,” Pham-Kanter said. Medical students given a Lipitor clipboard were more likely to prefer it over a generic cholesterol drug.

As Mansfield explains, “if you do something after getting a big gift, you can tell yourself you did that because of the gift. If you did something because of a small gift, your understanding is that you did it because you believed the person.”

Trump’s assertion that he will be able to separate his personal interest from the greater good is also not uncommon—and generally, false. “People have a ‘bias blind spot,’” says King, the Yale professor, “and are more likely to see bias in others than in themselves.”