What do some Supreme Court justices and physicians have in common? Both take gifts from those who stand to profit from their decisions, and both mistakenly think they can’t be swayed by those gifts.

Gifts are not only tokens of regard; they are the grease and the glue that help maintain a relationship. That’s not always unhealthy, but it’s important to note that gifts create obligation. The indebtedness of the recipient to the giver is a social norm in all cultures, and a basic principle of human interaction—something the French sociologist Marcel Mauss wrote about in his classic essay The Gift.

[Read: Gift-giving is about the buyer not the receiver]

This sense of reciprocity is subconscious and powerful, and doesn’t necessarily require a quid pro quo. In other words, a material gift need not be reciprocated as a material gift, but may be reciprocated in other ways, including a more favorable bent toward a company, a group, or a person.

The use of gifts to manipulate feelings and choices has interested me for decades. I’m a physician and professor who heads PharmedOut, a research group at Georgetown University Medical Center that studies covert pharmaceutical and medical-device marketing practices. I have sometimes been asked to serve as a paid expert witness on behalf of plaintiffs in litigation regarding pharmaceutical marketing.

Although my research focuses on how pharmaceutical companies subtly manipulate physicians’ beliefs about drugs and diseases, we now see questions about gift-giving playing out at the nation’s highest court. Recently, ProPublica reported on at least 38 destination vacations Justice Clarence Thomas has been treated to by “benefactors who share the ideology that drives his jurisprudence.” The week before, The New York Times revealed that Thomas had accepted—and failed to disclose—favorable financing for a high-end RV from a wealthy friend. And earlier this year, a ProPublica investigation found that Justice Thomas accepted many private plane trips, yacht cruises, and luxury vacations from the Republican donor Harlan Crow, a real-estate heir who has spent millions of dollars on ideological efforts to align the judiciary with his conservative beliefs.

In response to the controversy, Thomas released a statement saying, “Early in my tenure at the Court, I sought guidance from my colleagues and others in the judiciary, and was advised that this sort of personal hospitality from close personal friends, who did not have business before the Court, was not reportable.” Crow, for his part, stated, “We have never asked about a pending or lower court case, and Justice Thomas has never discussed one, and we have never sought to influence Justice Thomas on any legal or political issue.” Justice Samuel Alito made a similar argument. After being criticized for accepting luxury vacations from the billionaire Paul Singer—whose hedge fund was subsequently involved in at least 10 cases before the Court that Alito did not recuse himself from—Alito, in an op-ed, defended himself, saying, “On no occasion have we discussed the activities of his businesses, and we have never talked about any case or issue before the Court.”

But influence is rarely so crude, so obvious. Meals, trips, exclusive accommodations—gifts like these can foster a sense of gratitude in the gift recipient and, over time, increase his or her receptivity to the gift giver’s interests. And Crow’s defense—that “Justice Thomas and Ginni never asked for any of this hospitality”—is beside the point. Of course they didn’t ask. They didn’t have to.

As a friend of mine who went to work for a major foundation, where she would have decision-making power over which organizations would receive her foundation’s funding, was told by a colleague, “Say goodbye to true friends and bad meals.” Would that someone had said something similar to the justices.

Gifts can affect behavior in ways that recipients are unaware of. One classic study demonstrated that giving someone a soft drink before asking them to purchase raffle tickets for a good cause resulted in more ticket sales—and this was the case even if the gift giver was presented as unlikable.

Alito, in his op-ed, emphasized the “modest” nature of the lodging Singer covered for him on a luxury fishing vacation: the place “was a comfortable but rustic facility. As I recall, the meals were homestyle fare.”

[Adam Serwer: Clarence Thomas is winning his war on transparency]

But as much research has demonstrated, modest, even tiny, gifts have an outsize influence. The pens, mugs, and other branded trinkets given to physicians by drug reps were designed not only to keep certain brand names “top of mind” but also to help maintain relationships. These “reminder” items were largely stopped in 2009, when pharmaceutical companies voluntarily gave them up during a fleeting period of public censure. It was a commercial sacrifice, however, because small gifts are effective marketing.

My group’s research shows that accepting even one meal from a pharmaceutical company can result in physicians prescribing pricier drug options—and more drugs per patient. Many other studies have shown that the industry’s promotional tactics increase the prescription of targeted drugs. Tellingly, physicians acknowledge that promotion can affect other physicians’ choice of drugs, but think that they themselves are the exception. In other words, only their colleagues are susceptible to industry influence. In social psychology, self-serving bias is called “the bias blind spot.” We all more readily identify cognitive and motivational biases in others than in ourselves.

But the truth is that none of us is immune to persuasion tactics. Professionals, whether physicians or justices, may believe that because they are professionals, they are therefore incapable of acting unprofessionally. To believe that one is capable of unprofessional behavior leads to cognitive dissonance, essentially the discomfort produced by holding two opposing beliefs simultaneously (I am a professional/I am acting unprofessionally). Cognitive dissonance is reduced by rationalizing the dissonance, rejecting the significance of one of the factors contributing to the conflict, or, most painfully (so probably least done), eliminating the dissonance by changing one’s attitudes or behaviors.

Alito, in protesting that the seat on Singer’s private airplane would have otherwise gone unoccupied, gives us an excellent example of resolving cognitive dissonance by rationalization. The private plane trip is said to have been worth more than $100,000, which certainly puts it in the expensive-gift category. To resolve the dissonance between “I am an ethical Supreme Court justice” and “I accepted an expensive gift that could be considered a bribe,” Alito seems to have resolved the dissonance by discounting the value of the gift. As the plane presumably would have taken off anyway, Alito rejected the significance of what he was given, rationalizing that the trip would have cost the same whether or not one seat was occupied. In fact, that plane seat, occupied by a Supreme Court justice, was the most valuable seat on the plane.

Likewise, physicians rationalize their acceptance of industry meals and largesse, protesting, “My opinion cannot be bought by a free lunch.” But they are wrong. It is the very modesty of the pizza or sandwich that validates the physician’s belief that they are making an unbiased decision in prescribing a particular drug. Because it is unthinkable that their professional judgment could be affected by a Panera sandwich, the physician rationalizes that they are prescribing the targeted drug because they truly believe it is the best drug.

That bias blind spot might explain why Thomas, Alito, and many physicians see nothing wrong in their own choice to accept gifts from people who stand to benefit from their decisions. But the question gift recipients should ask themselves is whether the relationship would survive the absence of gifts, favors, services, and fawning. Would doctors even meet with drug reps if the reps were not dangling food, gifts, services, and income-enhancing opportunities, but offering only sales pitches? Would justices listen to billionaire pleasantries if they were meeting in a local coffee shop, with everyone paying for their own cappuccinos?

[Brooke Harrington: Mob justice]

A friend’s father, a businessman, had what he thought were close friendships with vendors he did business with; they went fishing, attended sports events, ate innumerable meals together. He was shocked, upon his retirement, to find the relationships abruptly terminated, the annual invitation to the Kentucky Derby missing, his “friends” permanently unavailable. Because the justices have a lifetime appointment, the people who would influence them have great incentive to play the long game. Unless they leave the bench, Supreme Court justices may never know who their real friends are.

The therapeutic choices physicians make affect large numbers of patients; the decisions made by Supreme Court justices affect the entire country. In both cases, those who would sway opinions for their own benefit must be distanced from those who make decisions that affect other people’s lives. The solution is easy, for both physicians and justices. All gifts, no matter how small, should be refused—or, better yet, banned.