 According to the standard definition, a social behaviour counts as altruistic if it reduces the fitness of the organism performing the behaviour, but boosts the fitness of others. This was the definition used by Hamilton (1964), and by many subsequent authors. However, there is less consensus on how to describe behaviours that boost the fitness of others but also boost the fitness of the organism performing the behaviour. As West et al. (2007) note, such behaviours are sometimes termed ‘co-operative’, but this usage is not universal; others use ‘co-operation’ to refer to behaviour that boosts the fitness of others irrespective of its effect on self; while still others use ‘cooperation’ as a synonym for altruism. (Indeed, in the simple Prisoner's dilemma game above, the two strategies are usually called ‘co-operate’ and ‘defect’.) To avoid this confusion, West et al. (2007) suggest the term ‘mutual benefit’ for behaviours that benefit both self and other, while Sachs et al. (2004) suggest ‘byproduct benefit’.