Altruism: Why are some people selfless?

(CBS News) What makes one person stop to help someone in need, while another simply walks by?

A New York City police officer was recently caught on camera in a random act of kindness, and in the process, captured the hearts of people around the world.

NYPD officer buys shoes for homeless man
NYPD officer says fame over act of kindness "surreal"

So why is it that some people are more prone to generosity?

Child and adolescent psychologist Dr. Jennifer Hartstein talked to Rebecca Jarvis and Anthony Mason about whether altruism is an innate quality, or something that can be taught.

"Compassion is something that is innate in people," Hartstein said. "We're born with a certain level of compassion that drives us to being altruistic."

On a cold night in midtown Manhattan, NYPD Officer Larry DePrimo bought a pair of shoes for a barefoot, homeless man without knowing someone was photographing the exchange.

The image of DePrimo's kind gesture has been shared and re-shared -- thanks to social media -- making him an inspiration to millions, and giving parents an opportunity for a teachable moment.

"It's so easy to show our children what they can do, how they can reach out, how they can be helpful," Hartstein said. "And not to do it for the fame, but to do it because it's meaningful to you."

To watch the full report and interview with Dr. Jennifer Hartstein, click on the video player above.