Depends upon the stimulus, and the reward or cost of the action. Straightforward psychology.
i help people when i can. i've stopped, and pushed broken down cars out of the middle of the road, before the driver got killed. i've picked up peoples dropped wallets, and handed them back to them(when they didn't realize they had dropped them).
there was a bad accident on rt 676 about a year ago. it was about 1am. the honda was on it's roof, a chevy on it's side, and the cadi that caused the entire mess was on fire about 50 yards past the other two.
i think there were about 10 or us that stopped, and just ran to the different cars to try to help the people in them. those helping the people in the chevy got 2 people out, and the three of us helping the people in the honda got the passenger out, but could not move the driver. the driver of the cadi was running around threatening to shoot people.
the police and fire company got there pretty quickly, and asked us all to step back. that's the only time i can remember ever being in a possibly life saving position.......but there was no possibility of reward. didn't care. was a bad situation, and people needed help, and apparently a few other people thought the same thing.