Washington: Reading the popular Harry Potter books, which emphasise the value of tolerance and respect for difference, may lead Americans to have a lower opinion of the controversial Republican presidential nominee Donald Trump, a new US study has found.

Even when controlling for party identification, gender, education level, age, evangelical self-identification and social dominance orientation – all factors known to predict Americans’ attitudes towards Trump – the Harry Potter effect remained, researchers said.

The more books the participants read, the greater the effect, according to the study by Professor Diana Mutz, professor at the University of Pennsylvania in the US.

In the nineteenth century, Uncle Tom’s Cabin was widely credited with shifting public opinion against slavery, but to date, there has been sparse evidence that fictional stories, even very popular ones, can influence political opinion.

Evidence has largely come from laboratory experiments – for example, forcing people to read one of two stories – rather than observing real-world consumption of stories.

Harry Potter’s popularity, with more than 450 million copies sold worldwide, made such a study possible in the public as a whole, researchers said.

“Because Trump’s political views are widely viewed as opposed to the values espoused in the Harry Potter series, exposure to the series may play an influential role in influencing how Americans respond to Donald Trump,” Mutz said.

To test that explanation for the Harry Potter effect, Mutz focused on three core themes from Harry Potter: The value of tolerance and respect for difference; opposition to violence and punitiveness; and opposition to authoritarianism.

In each case, Trump’s messages are opposed to the lessons conveyed in the Harry Potter series and closer to that of Potter’s enemy, Lord Voldemort.

For example, Harry and his friends advocate for oppressed house-elves and oppose Voldemort’s quest for blood purity among wizards.

Trump, by contrast, has called for a temporary moratorium on Muslim immigration and made offensive comments about outgroups of all kinds, including women, Mexicans, Asians, and those with disabilities.

Mutz polled a sample of 1,142 Americans in 2014, and again this year, asking about their Harry Potter consumption, their attitudes on issues such as waterboarding, the death penalty, the treatment of Muslims and gays, and (in 2016 only) their feelings about Trump on a 0-100 scale.

Party affiliation did not affect the likelihood that a person had read the Harry Potter books, the study found.

Democrats, Republicans, and Independents have all read books written by J K Rowling in roughly equal numbers.

The study found that each Harry Potter book read lowered respondents’ evaluations of Trump by roughly 2-3 points on a 100 point scale.

“For someone who has read all seven books, the total impact could lower their estimation of Trump by 18 points out of 100. The size of this effect is on par with the impact of party identification on attitudes toward gays and Muslims,” Mutz said.