This snack has been approved by the Democratic Party. Photographer: Robyn Beck/AFP/Getty Images

To paraphrase an observation attributed to the late Senator Daniel P. Moynihan, people are entitled to their own opinions, not to their own facts. But on some politically charged issues, people's ideological commitments sometimes settle their judgments about questions of fact.

A revealing body of research, coming largely from Yale Law School professorDan Kahan, finds that "cultural cognition" shapes our reactions to science -- and that our values affect our assessment of purely factual claims, even in highly technical areas. As a result, Americans predictably polarize on factual questions involving, for example, the effects of gun control, nuclear waste disposal and nanotechnology.

Consider current debates over genetically modified organisms and climate change.

The strong majority of scientists (of course, not all) accept two propositions. First, GMOs generally don't pose serious threats to human health or the environment. Second, greenhouse gases are producing climate change, which does pose serious threats to human health and the environment. (This judgment is strongly reaffirmed in today's reportfrom the United Nations Intergovernmental Panel on Climate Change. My wife is the U.S. ambassador to the United Nations but she had no involvement with the climate report.)

With respect to GMOs, Democrats are far more likely than Republicans to reject the prevailing scientific judgment. With respect to climate change, Republicans are far more likely than Democrats to reject the prevailing scientific judgment.

The partisan divide is easy to demonstrate. Among national leaders, many Democrats are concerned about GMOs; relatively few Republican leaders share that concern. Of the 27 senators who recently voted to require labeling of GMOs, not one was a Republican.

Among ordinary citizens, a strong majority of Democratic voters believe that GMOs are unsafe. Republican voters are evenly divided on the safety question -- a higher level of concern than that of their elected representatives but much lower than that of Democratic voters.

In Congress, it is not exactly news that Democrats are far more likely than Republicans to support action to reduce greenhouse gas emissions. Just one example: In a 2013 Senate vote on a nonbinding resolution calling for a "fee on carbon pollution," Republicans were in unanimous opposition, while most Democrats were supportive.

In recent years, about 75 percent of Democratic voters have said that they worry "a great deal" or "a fair amount" about climate change. For Republican voters, the percentage has ranged from 30 percent to 40 percent.

What is the best explanation for the fact that Republicans are more inclined to follow scientific opinion for GMOs, while Democrats are more inclined to do so for greenhouse gases? There are three possibilities.

The first explanation points to the role of interest groups. On the Democratic side, the concerns are of course sincerely held, but well-organized groups have been lobbying hard against GMOs, and they have been able to intensify public objections. These groups, which include the organic food industry and Whole Foods Market Inc., have influence and credibility within the Democratic Party and would undoubtedly gain from mandatory labeling (which would harm their competitors).

With respect to climate change, by contrast, the most powerful economic interests (such as the coal industry) have far greater influence within the Republican Party. Environmental groups, pressing for control of greenhouse gases, carry weight mostly with Democrats.

A second explanation points to the effects of political "echo chambers." With respect to GMOs, some Democrats listen largely to one another, and their fears have become amplified as a result of internal discussions -- even if science is not on their side. For greenhouse gases, the same phenomenon is occurring among Republicans. Here as elsewhere, discussions among like-minded people increase confidence, extremism and polarization.

A third explanation is the most interesting, and it builds on Kahan's research. It points to the crucial role of pre-existing ideological commitments, which can, on particular issues, crowd out the effects of scientific findings.

Many Republicans are opposed, in principle, to government interference with free markets. They're inclined to be suspicious of scientific evidence that purports to justify that interference, especially in the environmental domain.

By contrast, many Democrats are willing to indulge the assumption that corporate efforts to interfere with nature are potentially dangerous, especially if those efforts involve chemicals, new technologies or pollution. Among Democrats, scientific claims about the risks associated with GMOs and greenhouse gases fall on receptive ears.

To be sure, values do not always crowd out science. With respect to depletion of the ozone layer, the scientific evidence has long been overwhelming, and it led to bipartisan support for the Montreal Protocol, signed by President Ronald Reagan in 1988.

Even so, there is no question that pre-existing values help to account for political polarization with respect to GMOs and greenhouse gases. Taken together with interest-group activity and the echo chamber effect, those values help explain why the leaders of our two major political parties are strongly inclined to accept the dominant view within the scientific community in one case -- and to reject it in another.


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Cass R Sunstein at

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Timothy L O'Brien at