In this paper, we analyze how kinship among family members affects intergenerational cooperation in a public good game. 165 individuals from 55 families, comprising three generations (youths, parents, and grandparents), play a public good game in three different treatments: one in which three members of the same family play each other (family), a second with the youth and two non-family members but preserving the previous generational structure (intergenerational), and a third in which three randomly-selected players play each other (random). We find that players contribute more to the public good when they play with other family members, than when they play with non-family members. This effect is present in all three generations, and is independent of the gender of the players. We also observe the significant result that older generations contribute more to the public good, relative to their children.