If you doubt the prevalence of birtherism, or if you suspect that beliefs about President Obama's birthplace are simply being confused with uncertainty, this poll from CBS and The New York Times may convince you otherwise:

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A plurality of surveyed Republicans said they thought Obama was born on foreign soil and is constitutionally ineligible to hold his office. A majority across all parties said he was born in the U.S.

This poll appears reliable. CBS/NY Times polls are conducted by CBS's polling unit, a respected and nonpartisan entity. Some birther polls have come from left-leaning pollsters, like the since-discredited Research 2000. More recently, Fox News released one conducted jointly by Republican and Democratic pollsters. But if you're skeptical about the methodologies employed by Democratic firms or Fox News, it's tough to impugn CBS. This poll is based on a sample of 1,224 adults nationwide interviewed live by phone April 15-20, with a +/- three percent margin of error, and a higher margin of error for oversampled partisan subgroups.

CBS asked the question directly. The exact wording, as CBS tells The Atlantic:

According to the Constitution, American Presidents must be "natural born citizens." Some people say Barack Obama was NOT born in the United States, but was born in another country. Do YOU think Barack Obama was born in the United States, or do you think he was born in another country?

The "no" population is not inflated. CBS does not press respondents who say they don't know or don't have an opinion, as some pollsters do. CBS accepts uncertainty as a response, and we should take the comparative "no" and "I don't know" populations as legitimate.

There's an argument to be made that, for people who haven't looked into Obama's birthplace, being uncertain is totally reasonable. If a pollster calls asking where Obama was born, and one hasn't read anything about it, what else can one say, other than "How should I know?"

Asserting positively that Obama was born outside the U.S. is different. And among the 534 Republicans in CBS's oversample group, more of them held this opinion than either of the two alternatives.