(HealthDay) -- It was the small square photo clipped to an applicant's resume that most influenced whether a woman would be hired. But there was a hidden catch: The pictures showed the same six women both before and after weight-loss surgery.
The end result: The "employers" in the study rated these six women more poorly when their photos were taken when they were obese.
For the research, published recently in the International Journal of Obesity, the 95 raters actually were New Zealand undergraduate students who weren't aware that weight bias was the real focus of the study.
"Clearly, these were not actual employers," said study co-author Janet Latner, an associate professor of psychology at the University of Hawaii. "But they are people who will enter the workforce, and the underlying prejudice that they're displaying could ultimately affect their decisions regarding future colleagues."
The study participants -- mostly female -- received information packages on "candidates" to evaluate for a potential managerial position. The bogus resumes were equivalent as far as skills, experience and education.
The women shown in the photos had been located on websites. To rule out other appearance-based biases, the six women were of similar ages, from 29 to 32, and of European descent.
Before surgery, the women's body-mass index (BMI) -- a measure of body fat based on weight and height -- ranged from 38 to 41. A BMI of 30 or more is considered obese. After surgery, the women's BMI ranged from 22 to 24, considered normal weight.
The student "employers" rated candidates for starting salary, leadership potential and likelihood of being hired.
Based on the ratings, larger women had less chance of being hired. And if they managed to pass that hurdle, they still would have faced lower salaries and limited career progression.
The more attractive the raters considered themselves, the stronger the weight bias they displayed, researchers found. Having personality traits such as authoritarianism also was associated with being more biased.
Males weren't evaluated in the study, which leaves open the question of whether obese men face a similar bias.
"Men certainly face discrimination as well, but the research shows that they have to get to a higher weight in order for their weight to be consequential," said Michaela Null, a doctoral candidate in sociology at Purdue University in West Lafayette, Ind., whose research focus is fat studies.
Such hiring bias isn't just a hypothetical. In 2011, a Texas hospital instituted a policy that job candidates had to have a BMI of less than 35, but the hospital withdrew the policy in April.
"We sent a letter to [the hospital] that this was not acceptable," said James Zervios, director of communications for the Obesity Action Coalition. "The CEO spoke with our president and said the policy wouldn't be used. It was short and sweet. They realized they had done wrong, and they reversed it and did the right thing."
Only Michigan has a state law prohibiting discrimination on the basis of weight, along with several municipalities, said Justine Lisser, a senior attorney and adviser at the U.S. Equal Employment Opportunity Commission.
The Americans With Disabilities Act prohibits discrimination against the morbidly obese, Lisser said. Defined as a BMI of 40 or higher, morbid obesity contributes to a variety of medical conditions.
"There are, however, people who might be 'fat' or who may have a BMI that is higher than what is considered healthy, but not so high as to constitute morbid obesity," she added. "Unfortunately, these people would not be covered by our laws prohibiting discrimination because they would not have a covered disability."
"The entire thrust of [the Equal Employment Opportunity Commission's] mission is to have people considered for employment based on their qualifications and experience -- not on irrelevant factors," Lisser said.
Null described weight bias as a cultural problem, intertwined with issues of race, gender, poverty and class.
"You also have to look at before-hiring practices -- at the kinds of social networks people have, especially in today's economy, where you're more likely to get a job based on if someone recommended you," she said. "If you think about the stigma associated with [obesity] -- if you know that's not going to reflect positively on you, you may not recommend that person."
"Obesity prejudice is really the last acceptable form of prejudice," said Latner. "There needs to be legislation in place to protect obese workers from discrimination. We really need to protect that part of our workforce."
Explore further: Obesity affects job prospects for women, study finds
The Obesity Action Coalition has more on weight bias and stigma.