The problem with collective activism, according to the American economist Olson in 1965 AD, is that coercion or some incentive must be present in order to get a group of individuals to act in the common interest. In the latter half of the 20th Century, a number of activist leaders (Gandhi, King, Mandela, etc.) called for non-violent collective actions – boycotts, rallies, strikes, sit-ins, marches, petitions, educational programs, and such – to effect social, economic, political, and environmental changes. In industrial nations with semi-democratic governments, these have proven effective (sometimes). Collective activism has now gone viral, with social media and the internet now encouraging public support (financial, if nothing else) for these movements.