Most diversity training efforts at American companies are ineffective and even counterproductive in increasing the number of women and minorities in managerial positions, according to an analysis that turns decades of conventional wisdom, government policy and court rulings on their head.

A comprehensive review of 31 years of data from 830 mid-size to large U.S. workplaces found that the kind of diversity training exercises offered at most firms were followed by a 7.5 percent drop in the number of women in management. The number of black, female managers fell by 10 percent, and the number of black men in top positions fell by 12 percent. Similar effects were seen for Latinos and Asians.

The analysis did not find that all diversity training is useless. Rather, it showed that mandatory programs -- often undertaken mainly with an eye to avoiding liability in discrimination lawsuits -- were the problem. When diversity training is voluntary and undertaken to advance a company's business goals, it was associated with increased diversity in management.

The origins of diversity training trace back to the civil rights movement and the belief that education, sensitivity and awareness are key to reducing discrimination. While many companies have embraced such training as a way to make workplaces more inclusive and to cater to an increasingly diverse customer base, trainers and researchers note that other companies use "sensitivity training" superficially -- as a cosmetic response to complaints from internal and external critics.

Today, U.S. businesses spend from $200 million to $300 million a year on diversity training, but the new study is one of the first attempts to systematically analyze its impact. What it found is that programs work best when they are voluntary and focus on specific organizational skills, such as establishing mentoring relationships and giving women and minorities a chance to prove their worth in high-profile roles.

"When attendance is voluntary, diversity training is followed by an increase in managerial diversity," said Alexandra Kalev, a sociologist at the University of Arizona, who led the research. "Most employers, however, force their managers and workers to go through training, and this is the least effective option in terms of increasing diversity. . . . Forcing people to go through training creates a backlash against diversity."

Kalev said many trainers and executives told her they were not surprised by her findings. What this means, she said, is that many companies are not just pursuing poor policies, but are doing so even though their own experts know the training is ineffective or counterproductive.

Several experts offered two reasons for this: The first is that businesses are responding rationally to the legal environment, since several Supreme Court rulings have held that companies with mandatory diversity training are in a stronger position if they face a discrimination lawsuit. Second, many companies -- with the implicit cooperation of diversity trainers -- find it easier to offer exercises that serve public relations goals, rather than to embrace real change.

"They are more symbolic than substantive," said Lauren Edelman, a University of California professor of law and sociology, who independently reviewed Kalev's study. "It is a response to the general legal environment and the fact organizations copy one another."

Longtime diversity trainer Billy Vaughn said the results match what he has seen in practice. Vaughn is the co-founder of the national firm Diversity Training University International, which has been hired by organizations including wireless phone giant Qualcomm and the Central Intelligence Agency.

"If they are doing it for legal protection, they don't care" whether the training works, he said. It was hardly surprising that training could have counterproductive effects, he added, when the attitude often is, "Just do it, and just do it as cheaply as possible."

Kalev's latest research, which is not yet published, is the second comprehensive analysis that she and her colleagues have done. Her initial study, published in 2006 in the American Sociological Review when she was at the University of California at Berkeley, was the first systematic assessment of diversity training. It found that such training had minimal benefits.