This article investigates selfish behavior in games where players are embedded in a social
context. A framework is presented which allows us to measure the Windfall of Friendship,
i.e., how much players benefit (compared to purely selfish environments) if they care about
the welfare of their friends in the social network.
As a case study, a virus inoculation game is examined. We analyze the Nash equilibria
and show that the Windfall of Friendship can never be negative. However, we find that
if the valuation of a friend is independent of the total number of friends, the social welfare
may not increase monotonically with the extent to which players care for each other;
intriguingly, in the corresponding scenario where the relative importance of a friend
declines, the Windfall is monotonic again.
This article also studies convergence of best-response sequences. It turns out that in
social networks, convergence times are typically higher and hence constitute a price of
friendship. While such phenomena may be known on an anecdotal level, our framework
allows us to quantify these effects analytically. Our formal insights on the worst case equilibria
are complemented by simulations on larger social graphs, shedding light on robustness
and fairness aspects, as well as on the structure of other equilibria.