Do Brand Boycotts Make A Difference?

Whenever a massive social or political crisis occurs, furor tends to be directed with helpful intention at trying to support whichever cause or whoever is the target of the crisis. One way in which this manifests are boycotts of brands who are accused of supporting the oppressing party, whether the accusations are true or not. But is this really the most effective way of fighting for social justice? What is the actual effect of a boycott?