Yesterday, Pew Research Center released the results of a poll of US residents that asked about their acceptance of the theory of evolution. In keeping with past surveys, this one found that a completely uncontroversial idea within the scientific community—modern organisms are the result of evolution—is rejected by a third of the US public. While that fraction has held steady over time, the survey found that the political divide over evolution has grown over the past four years, with Republicans now even more likely to reject the idea than they were before.

In the poll, people were asked whether they thought that humans and other living things have existed in their present form since the beginning of time, or if we and other creatures had evolved over time. To make sure that mentioning humans didn't make things overly personal, Pew also asked a subset of questions just about other animals; this didn't make any difference in responses.

Acceptance of evolution was higher in younger people and those who had graduated college, as had been found in previous polling. Among the 60 percent of Americans who do accept the theory, a bit over half ascribed it solely to natural causes—32 percent of the total. 25 percent of all adults believed in some form of theistic evolution, where a deity or deities guided the process, possibly in a way that's indistinguishable from the random mutations that have been observed. That figure's a bit higher in most religious groups, and a bit lower among the unaffiliated.

Evangelicals were the most likely to reject evolution (nearly two-thirds of them did), yet most of the 27 percent who did accept it thought it happened due to natural causes. This might be explained by the idea that rejection of evolution is a way for people to reinforce their cultural affinity with groups they feel will also reject evolution. If someone is already willing to forgo those cultural ties, then they may be more open to other ideas that are atypical of their cultural group.

As in previous polls, this one found a partisan divide on the issue, with Republicans far less likely to accept the idea than independents or Democrats. And, based on differences with a similar poll performed in 2009, it appears the partisan divide is growing larger. Evolution is accepted by about two-thirds of Democrats and independents, and those numbers have been roughly steady over the past four years, with all results falling within the poll's three percent margin of error. Among Republicans, however, acceptance has dropped from 54 percent to 43 percent in that short period of time.

There are several potential explanations for this, most centered around the GOP's internal debate regarding ideological purity. That debate could either cause committed members to feel the need to reinforce their cultural ties with other Republicans or drive away those who already differ from the average party member in some regard. Without further study, it's difficult to determine what's causing this shift.

More details of the poll are available at Pew's site.