Group differences in broadness of values may drive dynamics of public opinion on moral issues

• In the US, moral norms show a long-term trend of change in the liberal direction.
• We suggest an explanation based on moral foundations theory.
• According to this theory, some moral foundations are unique to conservatives.
• Therefore, liberal arguments are more likely to sway a conservative than vice versa.
• We investigate the implied social dynamics in a mathematical model.

Here we propose the idea that the success of an argument in favor of an issue position should depend on whether the argument resonates with the audience’s values. Now consider two groups, one of which has a broader set of values than the other. We develop a mathematical model to investigate how this difference in broadness of values may drive a change on the population level towards positions in line with the more narrow set of values. The model is motivated by the empirical finding that conservative morality rests equally on moral foundations that are individualizing (harm and fairness) and binding (purity, authority, and ingroup), whereas liberal morality relies mainly on the individualizing moral foundations. The model then predicts that, under certain conditions, the whole population will tend to move towards positions on moral issues (e.g., same-sex marriage) that are supported by individualizing moral foundations.