In the 19th century, many American cities banned public appearances by “unsightly” individuals. Chicago’s ordinance was typical: “Any person who is diseased, maimed, mutilated, or in any way deformed, so as to be an unsightly or disgusting subject shall not expose himself to public view, under the penalty of a fine of $1 for each offense.”

Although the government is no longer in the business of enforcing such discrimination, it still allows businesses, schools and other organizations to indulge their own prejudices.

Over the past half-century, the United States has expanded protections against discrimination to include race, religion, sex, age, disability and, in a growing number of jurisdictions, sexual orientation. Yet bias based on appearance remains perfectly permissible in all but one state and six cities and counties. Across the rest of the country, looks are the last bastion of acceptable bigotry.

We all know that appearance matters, but the price of prejudice can be steeper than we often assume. In Texas in 1994, an obese woman was rejected for a job as a bus driver when a company doctor assumed she was not up to the task after watching her “waddling down the hall.” He did not do any agility tests to determine whether she was, as the company would later claim, unfit to evacuate the bus after an accident.

In New Jersey in 2005, one of the Borgata Hotel Casino’s “Borgata babe” cocktail waitresses went from a size 4 to a size 6 because of a thyroid condition. When the waitress, whose contract required her to keep “an hourglass figure” that was “height and weight appropriate,” requested a larger uniform, she was turned down.

“Borgata babes don’t go up in size,” she was told. (Unless, the waitress noted, they have breast implants, which the casino happily accommodated with paid medical leave and a bigger bustier.)

And in California in 2001, Jennifer Portnick, a 240-pound aerobics instructor, was denied a franchise by Jazzercise, a national fitness chain. Jazzercise explained that its image demanded instructors who are “fit” and “toned.” Portnick was both: She worked out six days a week, taught back-to-back classes and had no shortage of willing students.

Such cases are common. In a survey by the National Association to Advance Fat Acceptance, 62 percent of its overweight female members and 42 percent of its overweight male members said they had been turned down for jobs because of their weight. And it isn’t just weight that’s at issue; it’s appearance overall. According to a national poll by the Employment Law Alliance in 2005, 16 percent of workers reported being victims of appearance discrimination more generally.

Conventional wisdom holds that beauty is in the eye of the beholder, but most beholders tend to agree on what is beautiful. Researchers have found that when people are asked to rate an individual’s attractiveness, their responses are quite consistent. Facial symmetry and unblemished skin are universally admired. Men get a bump for height; women are favored if they have hourglass figures; racial minorities get points for light skin color, European facial characteristics and conventionally “white” hairstyles.

Among the key findings of a quarter-century’s worth of research: Unattractive people are less likely to be hired and promoted, and they earn lower salaries, even in fields in which looks have no obvious relationship to professional duties.

When researchers ask people to evaluate written essays, the same material receives lower ratings for ideas, style and creativity when an accompanying photograph shows a less attractive author. Good-looking professors get better course evaluations from students; teachers in turn rate good-looking students as more intelligent.

Not even justice is blind. In studies that simulate legal proceedings, unattractive plaintiffs receive lower damage awards. And in a study released this month, Stephen Ceci and Justin Gunnell, researchers at Cornell University, gave students case studies involving real criminal defendants and asked them to come to a verdict and a punishment for each. The students gave unattractive defendants prison sentences that were, on average, 22 months longer than those they gave to attractive defendants.

Just like racial or gender discrimination, discrimination based on irrelevant physical characteristics reinforces invidious stereotypes and undermines equal-opportunity principles based on merit and performance. And when grooming choices come into play, such bias can also restrict personal freedom.

Consider Nikki Youngblood, a lesbian who in 2001 was denied a photo in her Tampa high school yearbook because she would not pose in a scoop-necked dress. Youngblood was “not a rebellious kid,” her lawyer explained. “She simply wanted to appear in her yearbook as herself, not as a fluffed-up stereotype of what school administrators thought she should look like.”

So why not simply ban discrimination based on appearance?

Employers often argue that attractiveness is job-related; their workers’ appearance, they say, can affect the company’s image and its profitability. In this way, the Borgata blamed its weight limits on market demands. Customers, according to a spokesperson, like being served by an attractive waitress.

Such practices can violate the law if they disproportionately exclude groups already protected by civil rights statutes. Abercrombie & Fitch’s notorious efforts to project what it called a “classic American” look led to a race discrimination settlement on behalf of minority job seekers who said they were turned down for positions on the sales floor. But unless the victims of appearance bias belong to groups already protected by civil rights laws, they have no legal remedy.

One state (Michigan) and six jurisdictions (Washington, D.C.; Howard County, Md.; San Francisco; Santa Cruz, Calif.; Madison, Wis.; and Urbana, Ill.) have banned such discrimination. Some of the laws date back to the 1970s and 1980s, while some are more recent; some cover height and weight only, while others cover looks broadly; all make exceptions for reasonable business needs.

These cities and counties each receive zero to nine complaints a year, while the entire state of Michigan totals about 30, with fewer than one a year ending up in court.

Although the laws are unevenly enforced, they have had a positive effect by publicizing and remedying the worst abuses. Because Portnick, the 240-pound aerobics instructor who was turned away by Jazzercise, lived in San Francisco, she was able to bring a claim against the company. After a wave of sympathetic media coverage, Jazzercise changed its policy.

Given the stigma attached to unattractiveness, few will want to claim that status in public litigation. And in the vast majority of cases, the cost of suing and the difficulty of proving discrimination are likely to be prohibitive.

But stricter anti-discrimination laws could play a modest role in advancing healthier and more inclusive ideals of attractiveness. At the very least, such laws could reflect our principles of equal opportunity and raise our collective consciousness when we fall short.

Deborah L. Rhode is a law professor at Stanford University and the author of “The Beauty Bias: The Injustice of Appearance in Life and Law.”