Returns the number of points (payoff) that a focal player accumlates from one round of a game with an opponent given a payoff vector given a decision "C" or "D" for each player
PD(a1_play, a2_play, payoffs)The play choice of the focal player (0 or 1)
The play choice of the opponent player (0 or 1)
A vector of payoffs (length = 4) to the focal player as a consequence of both players playing 0 (payoffs[1]), the focal player only playing 0 (payoffs[2]), the focal player only playing 1 (payoffs[3]), and both players playing 1 (payoffs[4])
fitness The payoff to the focal player (points accumulated from the interaction)