  In a function that takes its inputs from various players, the effect of a
player measures the variation he can cause in the expectation of that function.
In this paper we prove a tight upper bound on the number of players with large
effect, a bound that holds even when the players' inputs are only known to be
pairwise independent. We also study the effect of a set of players, and show
that there always exists a "small" set that, when eliminated, leaves every set
with little effect. Finally, we ask whether there always exists a player with
positive effect. We answer this question differently in various scenarios,
depending on the properties of the function and the distribution of players'
inputs. More specifically, we show that if the function is non-monotone or the
distribution is only known to be pairwise independent, then it is possible that
all players have 0 effect. If the distribution is pairwise independent with
minimal support, on the other hand, then there must exist a player with "large"
effect.
