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How come today's conservatives are more liberal than yesterday's liberals? Why has public opinion in large parts of the world shifted so rapidly in favour of gay and lesbian rights, but been virtually unchanged on other contested issues such as abortion rights? A study from a Swedish team of researchers recently published in the social science journal Nature Human Behaviour answers several critical questions regarding how public opinion changes on moral issues, and have created a scientific model that can predict such public opinion changes.

"Our study shows that the connection between a certain moral position and the type of argument that is raised in its defence can predict which opinions will gain ground," says Pontus Strimling, research leader at Institute for Futures Studies in Stockholm, Sweden.

The four researchers—mathematicians, psychologists and social scientists at the Institute for Futures Studies and Stockholm University—have built mathematical models based on new findings within moral psychology and used them to predict opinion changes on moral issues over time. The predictions were then compared with over 40 years of data on public opinion. Their conclusion is that the key characteristic of opinions that gain ground is that they are supported by arguments about what is fair and what does not cause harm to others.

"The connection is very clear. And the model can be used to make qualified assessments about the future," says Pontus Strimling.

Opinions based on other classical grounds used to determine right and wrong actions—loyalty, authority, purity, religion—can gain support temporarily, but over time, opinions based on these arguments lose support all over the political spectrum. The stronger the connection an opinion has to arguments about fairness and harm, the greater the probability that it will gain ground in public opinion. Also, the stronger the connection is, the faster the change will come.

"This can explain why public opinion has changed so rapidly in favour of gay and lesbian rights. Arguments in favour of same-sex marriage, for instance, are based on principles of fairness, while arguments against are based on authority and purity. Over time, the latter arguments lose support. On other issues such as support for active euthanasia or to ban pornography, there are powerful arguments on both sides, so change goes slower," says Pontus Strimling.

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More information: Pontus Strimling et al, The connection between moral positions and moral arguments drives opinion change, Nature Human Behaviour (2019). Journal information: Nature Human Behaviour Pontus Strimling et al, The connection between moral positions and moral arguments drives opinion change,(2019). DOI: 10.1038/s41562-019-0647-x