Appendix

Appendix 1: Treatment wording

Shown to all treatment groups:

Senator James Inhofe (R-OK), Chairman of the Senate Environment Committee, stated in an interview last month that environmental regulations aimed at addressing climate change were “alarmist” and that the science over climate change remains unsettled.

“It's important to question whether climate change is even a problem for human existence,” Inhofe said. “Thus far no one has seriously demonstrated any scientific proof that increased global temperatures would lead to the catastrophes predicted by alarmists. The claim that global warming is caused by man-made emissions is simply untrue and not based on sound science.”

Inhofe has made similar comments in the past, most notably during a Senate speech in 2015 when he argued that the earth is currently in a cooling period. Inhofe has been a strong critic of recent federal bills aiming to cut carbon pollution by regulating power plant emissions and oil drilling. The regulations pose a serious threat to the country’s economy, he said.

Treatment 1: Only received misinformation article with no correction.Footnote 8

Treatment 2: Misinformation article with the following (scientist source) conclusion:

However, several scientists have been critical of Inhofe’s remarks on climate change. Members of NASA pointed to recent reports from the Intergovernmental Panel on Climate Change (IPCC) that recently declared that human activities have caused most of earth’s temperature rise since 1950 and will continue to do so in the future. The IPCC, which is a non-partisan panel of scientists from over 100 countries, estimates that global temperatures may increase by as much as 4.8 °C over pre-industrial levels by 2100. Dr. James Hansen, a climatologist who worked at NASA for over 40 years said, “When you have over 90 percent of the world’s scientists who have studied this stating that climate change is occurring and that humans play a contributing role, it's time to defer to the experts.”

Treatment 3: Misinformation article with the following (Democrat source) conclusion:

However, several Democrats have been critical of Inhofe’s remarks on climate change. Senators Brian Schatz (D-HI) and Maria Cantwell (D-WA) pointed to recent reports from the Intergovernmental Panel on Climate Change (IPCC) that recently declared that human activities have caused most of earth’s temperature rise since 1950 and will continue to do so in the future. The IPCC, which is a non-partisan panel of scientists from over 100 countries, estimates that global temperatures may increase by as much as 4.8 °C over pre-industrial levels by 2100. Democrat Senator Sheldon Whitehouse (D-RI) said, “When you have over 90 percent of the world’s scientists who have studied this stating that climate change is occurring and that humans play a contributing role, it's time to defer to the experts.”

Treatment 4: Misinformation article with the following (Republican source) conclusion:

However, several Republicans have recently been critical of Inhofe’s remarks on climate change. Senators John Thune (R-SD) and Kelly Ayotte (R-NH) pointed to recent reports from the Intergovernmental Panel on Climate Change (IPCC) that recently declared that human activities have caused most of earth’s temperature rise since 1950 and will continue to do so in the future. The IPCC, which is a non-partisan panel of scientists from over 100 countries, estimates that global temperatures may increase by as much as 4.8 °C over pre-industrial levels by 2100. Republican Congressman Chris Smith (R-NJ) said, “When you have over 90 percent of the world’s scientists who have studied this stating that climate change is occurring and that humans play a contributing role, it's time to defer to the experts.”

Appendix 2: Question wording

Survey questions

Outcome variables:

Please state your level of agreement with the following statements on a scale of 1–10, 10 indicating the strongest level of agreement.

Acknowledgement of scientific consensus on climate change

There is a general consensus among scientists that the average temperature on earth has been getting warmer over the past few decades.

Strongly disagree... 10. Strongly agree

Human activity affects climate change

The problem of climate change is mainly due to human activity such as burning fossil fuels.

Strongly disagree... 10. Strongly agree

Seriousness of climate change

How important a problem do you think climate change is at this moment, on a scale of 1–10?

1: Not important at all ... 10: Very important

Attention check

Which of the following statements best summarizes the article that you just read?

a. Senator James Inhofe believes that climate change is an urgent problem b. Senator James Inhofe is skeptical about climate change [correct answer] c. Senator James Inhofe is optimistic about the 2016 election d. Senator James Inhofe wants bipartisan policy on climate change

Respondents who did not answer (b) to the above question were considered to have not adequately read the assigned treatment and were removed from the sample.

Appendix 3: Demographics

Variable Mechanical Turk sample Sex Male 51% Female 49% Race White 81% Asian 7% Hispanic 5% Black 6% Education Up to high school 12% Some college studies, no degree 36% Four year college degree 37% Postgraduate degree 15% Partisanship Republican 20% Democrat 40% Independent 31% N 1338

Demographics across treatments

Treatment type Variable Null (denial) Expert Democrat Republican Male 53% 52% 52% 47% White 83% 81% 76% 83% College or higher 50% 57% 48% 52% Republican 17% 18% 22% 20% Democrat 38% 39% 36% 47%

Sample Ideology breakdown (percentage by party identification)

Republican Democrat Independent Total Very conservative 16.8 0.3 1.3 66 Conservative 54.3 2.7 6.0 230 Moderate 25.3 19.8 71.1 399 Liberal 3.0 52.9 16.6 424 Very liberal 0.6 24.3 5.1 186 100 100 100 1305

Appendix 4: Summary of pilot study

A preliminary/pilot study was conducted on Amazon Mechanical Turk in April 2015. Participants in this study were randomly assigned one of the following treatments, with a post-treatment survey. While the results from the two experiments are largely consistent in terms of the significance of effects, this design failed to control for (a) the content of the correction message, and (b) inadequate isolation of personal affect or credibility for the correction sources dues to national name recognition.

Shown to all treatment groups:

In an interview with the Cincinnati Enquirer, Senate Majority Leader Mitch McConnell told the editorial board that he does not believe in climate change, and that the science over climate change remains unsettled. McConnell has raised doubts about climate change in the past, but never as directly.

“I don't buy that climate is changing. In 1970s, we were all concerned about the ice age coming,” said the Republican senator, who comes from Kentucky, and was unanimously elected to be Senate Majority Leader last month.

McConnell said that the science remains unsettled on the problem of climate change. “Pick your period and data will support your bias,” said McConnell, indicating that the earth has gone through cooling and warming periods in the past. “Each side has their scientists, and they can all go in and argue,” he responded, when asked whether he believed if human activity contributes to global warming. McConnell also criticized newly proposed Environmental Protection Agency rules that would cut carbon pollution from coal power plants. The regulations pose a serious threat to Kentucky’s coal industry, he said.

Treatment 1: Only received denial article with no correction.

Treatment 2: Denial article with the following conclusion:

McConnell’s statements contradict findings from the most recent report published by the Intergovernmental Panel on Climate Change (IPCC.) The panel, which consists of thousands of scientists and reviewers from more than 100 countries, confirmed with 95% certainty that human activities have caused most of earth’s temperature rise since 1950, and will continue to do so in the future. The IPCC’s most recent report, released on 2 November 2014 in Copenhagen, estimates that global temperatures may increase by as much as 4.8 °C over pre-industrial levels by 2100.

Treatment 3: Denial article with the following conclusion:

McConnell’s statements were refuted by several members of the Democrat Party, including Senators Elizabeth Warren, Brian Schatz, and Minority Leader Harry Reid. In an earlier statement, Schatz (D-Hawaii) had called climate change “the most pressing issue of our time,” declaring that there was overwhelming evidence that “it is real, it is caused by humans, it is happening now, and it is solvable.”

Treatment 4: Denial article with the following conclusion:

Although many other Republican senators have been skeptical of climate change, a growing number have become more accepting of the issue and its risks in the past year. Marco Rubio (R-FL) and John Thune (R-SD) have in recent months publicly acknowledged that global temperatures are increasing, while Chris Christie (R-NJ) said two years ago that “When you have over 90 percent of the world’s scientists who have studied this stating that climate change is occurring and that humans play a contributing role, it's time to defer to the experts.”

Respondents were asked similarly worded questions about whether climate change is occurring and whether it is affected by human activity. A similar attention check was included to ensure respondents had read the treatment. A total of 969 respondents completed this survey. Results showed that a Republican correction was effective in persuading other Republicans to update their reported attitudes on climate change closer to the scientific consensus, and had moderate effects on Independents and Democrats. In an effort to rely on actual quotes from real politicians, the details of corrective messages differed in this pilot study. This sacrificed some degree of internal validity for external validity, as the intensity and phrasing of messages differed by group. We then developed similar corrective treatments for the main study discussed in the manuscript, but with a greater focus on maintaining internal validity.

Mean opinions by treatment group and partisanship from pilot

Partisanship Corrective treatments: None (denial) Expert (IPCC) Democrat Republican Belief in evidence that climate change exists Republican

(std. deviation)

(N) 6.392

(2.293)

56 7.195

(2.595)

46 6.510

(2.550)

49 7.609

(2.157)

64 Democrat 8.705

(1.711)

129 8.877

(1.588)

155 8.644

(1.681)

135 8.895

(1.646)

124 Independent 7.615

(1.914)

39 8.588

(1.760)

34 7.560

(2.083)

25 8.170

(2.301)

41 Combined 7.970

(2.098)

241 8.480

(1.975)

254 8.000

(2.187)

223 8.360

(1.979)

250 Belief that human activity affects climate change Republican

(std. deviation)

(N) 5.446

(2.507)

56 5.586

(2.963)

46 5.489

(2.534)

49 6.234

(2.758)

64 Democrat 8.364

(1.672)

129 8.380

(1.762)

155 7.925

(1.835)

135 7.967

(1.929)

124 Independent 5.846

(2.508)

39 6.941

(2.282)

34 6.360

(2.885)

25 7.073

(2.327)

41 Combined 7.149

(2.451)

241 7.610

(2.422)

254 7.179

(2.411)

223 7.308

(2.339)

250

Two-tailed t tests from this study:

Partisanship Corrective treatments (compared to the null/denial group) Expert (IPCC) Democrat Republican Belief in evidence that climate change exists Republican ∆ mean .802*

(1.657) .117

(0.248) 1.216***

(2.992) Democrat ∆ mean .171

(.877) −.060

(0.292) .189

(0.226) Independent ∆ mean .972***

(2.248) −.055

(1.070) .555

(0.389) Belief that human activity affects climate change Republican ∆ mean .140

(0.259) .0433

(0.088) .787**

(1.628) Democrat ∆ mean .016

(0.079) −.438**

(2.026) −.396*

(1.749) Independent ∆ mean 1.095**

(1.939) .513

(0.753) 1.227**

(2.269)

T-statistics in parentheses, * P < 0.10, ** P < 0.05, *** P < 0.01