Ad hominem arguments — attacking a person to disprove his or her claims — is considered a logical fallacy. But a new study published in PLOS One suggests that some ad hominem attacks can effectively erode people’s trust in scientific claims.

The research found that attacking the motives of scientists undermines the belief in a scientific claim just as much as attacking the science itself.

“Some people think ‘big pharma’ is responsible for a lot of pills that do nothing. Others think that major pharmaceutical companies produce a lot of safe and effective medicines. Some people distrust vaccines, while others place a great deal of trust in them. Some people accept human forced global warming, and others think it is a hoax,” said study author Ralph Barnes, an assistant professor of psychology at Montana State University.

“I think scientists don’t yet have a complete understanding of how the public reacts to scientific claims, and I wanted to contribute (even if in a small way) to that effort.”

After conducting two experiments with a total of 638 participants, Barnes and his colleagues found that some ad hominem attacks on scientific claims could be just as effective as attacks on the empirical foundations of the claims.

In the experiments, the participants read about a scientific claim that was attributed to a specific researcher along with an additional tidbit of information that either attacked the researcher or attacked the research methods behind the claim. They then indicated their attitude towards the truth of the claim.

Attacking the researcher’s motives produced more negative attitudes about the claims, but attacking the researcher’s competence did not.

Information about past misconduct related to the researcher and information about potential conflicts of interest had just as great an impact as attacks on the research itself. But information attacking the researcher’s education and information describing the researcher as having a bad reputation among scientists were not linked to more negative attitudes.

“One key finding is that if members of the general public are aware of a conflict of interest connected to a scientific finding, then this will seriously undermine their faith in that finding,” Barnes told PsyPost. “What the study does is allow us to quantitatively compare the amount of attitude change based on knowledge of conflict of interest to the amount of attitude change based on knowledge of outright research fraud and misconduct (such as faking the data).”

“What we see is that knowledge of conflict of interest is just as powerful as knowledge of research fraud.

“So who would be interested in this finding? Policy makers in the public and private sector. In particular: Journal editors-because they have to make decisions about conflict of interest disclosures. Those with decision making authority in private corporations-because they have to decide whether to rely on neutral research or in-house research. If they choose in-house research, they have to consider whether the conflict of interest inherent in the research will become general knowledge, or whether it will be known only to a select few. Policy makers at NIH, NSF, etc.-because they have to decide policy on conflicts of interest.”

But Barnes cautioned that people should not come to firm conclusions based on a single study.

“The paper is just a single publication containing two studies (the second is a replication of the first). The fact is, that you don’t settle anything in science with a single publication. I would like to see some other studies (preferably using different stimuli/methods/procedures) demonstrate the same effect we have shown,” he remarked.

“If it is the case that knowledge of conflict of interest has a powerful effect on the public’s view of science claims, then a number of organizations and institutions may want to react to that information. But for the time being, we need to wait for some form of replication.”

“I have spent fair amount of time reading books and websites attacking vaccines and drugs developed by big pharma. Though I haven’t bothered to keep a tally of every time that those attacking vaccines and big pharma mention conflicts of interest, it certainly seems like conflicts of interest are frequently mentioned,” Barnes added.

“If conflicts of interest are as powerful as the current studies suggest, that might indicate that some of the success of the groups attacking vaccines and mainstream medicine may be due to the fact that they are communicating conflict of interest information to their readers.”

The study, “The effect of ad hominem attacks on the evaluation of claims promoted by scientists“, was co-authored by Heather M. Johnston, Noah MacKenzie, Stephanie J. Tobin, and Chelsea M. Taglang.