25% of a group according to this study published in Science journal.

“A new study finds that when 25 percent of people in a group adopt a new social norm, it creates a tipping point where the entire group follows suit. This shows the direct causal effect of the size of a committed minority on its capacity to create social change.”

And this is encouraging and a key reason I do my blog:

“While shifting people’s underlying beliefs can be challenging, Centola’s results offer new evidence that a committed minority can change what behaviors are seen as socially acceptable, potentially leading to pro-social outcomes like reduced energy consumption, less sexual harassment in the workplace, and improved exercise habits.”