and though you may not think twice about these everyday moral decisions.

Calculations behind them can be pretty complicated as you're about to see in this next experiment.

Imagine you're walking through the park one afternoon.

When you see someone fall down,

would you lend a hand?

We've enlisted the help with an actress and set up a hidden-camera experiment to see what strangers will do in this exact scenario.

How do you think people react once our actress hits the pavement?

I think she may have hurt herself.

That didn't take long.

Within seconds, a crowd quickly gathers around ready and willing to help.

We run this experiment over and over,

and every time people responded to help her up in less than ten seconds.

Are you okay?

I'm okay.

It seems people lend a helping hand to someone lying on the ground

makes you feel good about humanity, right?

But is there anything we can do to make passersby turn a blind eye to the same person?

What happens when we change nothing but the way our actress looks?

Would you help someone up who looks so down and out?

How would people react now?

How long would it take for someone to help?

Thirty seconds have gone by, and no one has come close to help her.

They just keep walking.

More than a minute has passed, and no one has helped.

She's been on the ground for 15 minutes.

Before you judge any of these people too harshly,

ask yourself, what would you do in this situation?

It seems people's brains are having a hard time seeing this woman in need.

It is if she were invisible even to this couple sitting just ten feet away.

Why did so many people start to help this woman but not this woman?

Why does our actress's appearance make such a huge difference?

Empathy is an important motivational state for determining whether or not you will help someone in need.

But what triggers our sense of empathy for others?

Psychologists have long noted that we automatically categorize other people as members of our in group or out group.

Humans are a vaulting groups where our individual survival depended on the survival of others,

so it makes evolutionary sense to help people in our own groups.