Watching a strangerís bag can be self-rewarding because of the feeling of commitment and a sense of moral justice.
Of cause it is self rewarding and selfish. That is the whole point of there being altruistic drives. They are drives because they are rewarding but they cause "altruistic" behavior all the same in the sense of no possibility for physical benefit under conditions where you are not going to meet the person again.
In nature these drives have mostly evolved under condition where everyone you meet is someone you are going to meet again, and in-group/out-group adaptations and several other adaptations that result in non-altruistic behavior have rather evolved to limit our altruistic behavior from going to waste.
A new study from Harvard (
I know...) discusses how nice guys finish first.
Having taken a brief look over that Harvard study with it's laboratory testings it doesn't seem to be offering anything new that hasn't already been discovered.
It appears to be a kind of rip off from Game theory "Tit for Tat" from the 80's that brought publicity to many evolution theorists' work about reciprocal altruism like Robert Trivers' and especially ripping off more recent works of Paul Zak the founder of Neuroeconomy without giving them enough mention.(Not claiming blame on Harward just seeing how it appears to me)
Paul Zak makes interesting advancement in bringing in the functions of neurochemistry namely the role of Oxytocin in trust signals. Oxytocin besides being a hormone is also a neurotransmitter and also has a role in sex, commitment, pregnancy and child labor.
Game theory and neuroeconomy also relate to reciprocal altruism you mentioned earlier as the success of different game tactics is related to the number of different agents with different tactics and the balance of agents using different tactics is bound to appear in evolution. Simplified that is why most people are assertive, some are overly trusting and a small portion are downright ruthless.
In a world where every overly trusting person was assertive all the ruthless ones would die out and overly trusting ones would be bound to be born and succeed with a reputation of exceptional kindness among the population of the assertive, which in turn would create a successful breeding ground for new generation of downright ruthless people.
In a sense; con men praying on the gullible is a mathematical balance of the laws of nature. And the assertive ones will always be the most successful.