Promoting social causes online can mobilise millions and raise huge sums of money. But it only leads to long term changes if the campaigns don't fizzle out prematurely. Social psychologist, Sander van der Linden, University of Cambridge, thinks he's found the formula for social media campaigns to be effective in the long run, detailing what he calls "viral altruism" in a study published in Nature Human Behaviour. Ricky Nathvani went to go find out exactly what it is...