A very slender friend recently admitted to me that she “can’t stand to be around fat people.” Her reaction is almost visceral, and it prompts her to avoid social and professional contact with people who are seriously overweight. Although she can’t pinpoint the source of her feelings, she said they go back as far as she can remember.

And she is hardly alone. Decades ago, researchers found that weight-based bias, which is often accompanied by overt discrimination and bullying, can date back to childhood, sometimes as early as age 3.

The prejudiced feelings may not be apparent to those who hold them, yet they can strongly influence someone’s behavior. A new study by researchers at Duke University, for example, found that “implicit weight bias” in children ages 9 to 11 was as common as “implicit racial bias” is among adults.

The study’s lead author, Asheley C. Skinner, a public health researcher, said that prejudices that people are unaware of may predict their biased behaviors even better than explicit prejudice. She traced the origins of weight bias in young children and adolescents to the families they grow up in as well as society at large, which continues to project cultural ideals of ultra-slimness and blames people for being fat.

“It’s pretty common for parents to comment on their own weight issues and tell their children they shouldn’t be eating certain foods or remark about how much weight they’re gaining,” Dr. Skinner said.

Explicit weight bias is well documented, as are its damaging effects on people who struggle with their weight. Yet, implicit bias can also result in discrimination and socially undesirable behavior that negatively affect people who are seriously overweight.

Weight bias is widespread in society, occurring in employment, education, the media, health care and even in relationships with family members, parents and teachers, according to Dr. Scott Kahan, director of the National Center for Weight and Wellness in Washington, D.C.

“Obesity has been called the last socially acceptable form of prejudice, and persons with obesity are considered acceptable targets of stigma,” Dr. Kahan wrote in a 2015 blog post. He said that weight bias “occurs even in people who are otherwise fair-minded and nonjudgmental – even in obesity specialists,” who may not realize that it “predisposes to unhealthier behaviors and more weight gain.”

Whether explicit or implicit, weight-based bias can be counterproductive, impairing the ability of overweight people to lose weight and keep it off. Studies by Rebecca M. Puhl and colleagues at the University of Connecticut Rudd Center for Food Policy and Obesity, among others, have found that overweight and obese people who experience weight-based bias and who manage to lose weight are less able to maintain their weight loss.

Stigmatization is associated with more frequent binge eating and other “maladaptive eating patterns,” Dr. Puhl reported in a comprehensive review of the subject in the American Journal of Public Health. “In a study of more than 2,400 overweight and obese women who belonged to a weight loss support organization,” she wrote, “79 percent reported coping with weight stigma on multiple occasions by eating more food, and 75 percent reported coping by refusing to diet.”

Furthermore, experiencing weight stigma can result in a poor self-image, depression and stress that in turn increase the risk of poor eating habits and difficulty losing weight and keeping it off. People can internalize weight stigma, blaming themselves for their excess weight and the social discrimination they experience.

Even people who simply think they’re overweight – regardless of what they weigh – may be “at increased risk for weight gain and eating more in response to social threats,” Dr. Puhl wrote. Three long-term studies involving more than 14,000 adults in the United States and Britain showed that adults who thought of themselves as overweight were more likely to gain weight over time, regardless of what they originally weighed and whether their self-perception of being overweight was accurate.

When weight stigma is internalized, it significantly diminishes a person’s chances of maintaining weight loss over the long term, Dr. Puhl and colleagues confirmed in an online survey of 2,702 American adults.

A study by Robert A. Carels and colleagues at Bowling Green State University of 46 overweight and obese adults enrolled in a behavioral weight loss program found that both explicit and implicit weight stigmatization was linked to greater caloric intake, less exercise and energy expenditure, less weight loss and a greater likelihood of dropping out of the program.

“There are very visible people in society making comments about people’s physical appearance in very inappropriate ways,” Dr. Puhl noted in an interview. “Where are the voices saying that this is not acceptable? That silence communicates this is socially acceptable.”

Three states – New York, Maine and New Hampshire – have passed laws prohibiting discrimination against people based on their weight, Dr. Puhl said. And Congress has amended the Americans with Disabilities Act to protect those with “severe obesity” against discrimination in employment (although many people who are discriminated against because of their weight are not covered by this law).

There has also been a growing movement to improve affordable access to healthy foods in communities considered “food deserts” where obesity is often rampant.

Still, being overweight is one of the most, if not the most, common reason children are bullied, a problem sorely in need of intervention and prevention both in schools and organizations to head off self-image problems and eating disorders that result in lifelong weight struggles, Dr. Puhl said.

While the ideal solution to weight bias ultimately depends on education of both lay people and health professionals, people currently struggling with weight problems can’t wait for a society-wide reformation that may help to absolve them of personal responsibility for their weight.

“With extreme thinness being so prevalent in the media,” Dr. Puhl said, “ it’s hard to change societal attitudes.“

To compete with “all the well-funded messages from the diet and fashion industries,” she recommends making a concerted effort at self-acceptance and engaging in “positive self-talk” that challenges stereotypes to help people with weight issues recognize that what really matters to self-worth is “character, intelligence, ambition, effort and contributions to society.”

“We all need to move away from the current appearance-focused culture and recognize that other things matter more than what a person looks like,” she said.