A conservative researcher's 2009 dissertation, which argued that Hispanic immigrants to the U.S. have substantially lower IQs than whites, put one of the biggest opponents to an immigration reform bill in Congress on the defensive on Wednesday.

The dissertation by Jason Richwine, then a Ph.D. candidate at Harvard University, argues that "[n]o one knows whether Hispanics will ever reach IQ parity with whites, but the prediction that new Hispanic immigrants will have low-IQ children and grandchildren is difficult to argue against. From the perspective of Americans alive today, the low average IQ of Hispanics is effectively permanent."

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On page 88 of his dissertation, Richwine included a section on "the growing Hispanic underclass." Heritage's paper takes the position that undocumented immigrants who are currently in the country would be a burden on the U.S. safety net and welfare programs if they were to be given citizenship.

An underclass, Richwine wrote, is "a socially isolated group of people for whom crime, welfare, labor force dropout, and illegitimacy are normal aspects of life." He argues that his data shows that "Hispanic immigrants come [to the U.S.] to work, but their children's labor force participation slips considerably."

"Superior performance on basic economic indicators is to be expected from later generations, who go to American schools, learn English, and become better acquainted with the culture," Richwine wrote in the dissertation. "Despite built-in advantages, too many Hispanic natives are not adhering to standards of behavior that separate middle and working class neighborhoods from the barrio."

"There can be little dispute that post 1965-immigration has brought a larger and increasingly visible Hispanic underclass to the United States, yet the underlying reasons for its existence cannot be understood without considering IQ," Richwine wrote.

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Lower-IQ individuals, Richwine also wrote, are more likely to accept government handouts.

"When given the choice between a paycheck from a low-paying job and a welfare check, most intelligent people would realize that the welfare check offers them no potential for advancement. Low-IQ people do not internalize that fact nearly as well," he wrote.