And think of the collective grief in the United States that we all felt, that brought us all together, after 9 11.

In order to survive Humans have had to organise themselves.

You do find groups in the animal kingdom you do find cooperative groups but these groups are always either very small or they're all siblings.

A bunch of dirt?

I heard it's a pretty bad outbreak.

Shankar Vedantam is the author of The Hidden Brain

But when we talked to people, it appeared that a funnel effect had happened in these tribes all across the United States.

Size of the total population

They all disappeared!

And many of you were doing what great, if you will, tribal leaders do, which is to find someone who is a member of a tribe, and to find someone else who is another member of a different tribe, and make introductions.

And we have to duke it out every day, so we resort to politics.

Well, what do you think i've got here now?

English Subtitles by T. Kameneva

So look at it this way

Well, if that's true, a couple other things had to be true as well.

Population size

I think I feel a little full. The next movement is called mobs.

The Persian warrior elite.

And we know this because when we study different language groups and associate them with their cultures, we see that different languages slow the flow of ideas between groups.

The main argument against group selection has always been that, well sure, it would be nice to have a group of cooperators, but as soon as you have a group of cooperators, they're just going to get taken over by free riders, individuals that are going to exploit the hard work of the others.

And they took a group of physicians.

They share a nest.

Travel in packs, no way to tell.

The third is what you might call sympathy, the ability to work within groups.

Number of success overall

So I decide to replace Francois Fillon

People forget that. People think of trade as a modern thing.

Is nothing sacred anymore, for heaven's sake?

It's a whole race of people no bigger than ants.

But the good news is they are out there.

We had no idea

English Subtitles by T. Kameneva

Let's get a random sample.

Fourth, students must stop traveling in packs.

So for example, there was one study that was done in a population of Ashkenazi Jews in New York City.

I think it's kind of obvious and intuitive that you catch the moods of people around you, but now they're finding that people get fat in groups, they get happy in groups, they quit smoking in groups.

There's a bubble.

Now here is the net out of this.

But the question on the table is this

For I account your loss as my loss, and your grief as my own.

The crowd was moving in a solid body.

Now, the reason we knew that was because we spent the previous 10 years studying tribes, studying these naturally occurring groups.

You're headed for more trouble than you can possibly understand, Sean.

You can do it. So Paula Radcliffe, unfortunately, does it.

However, not everyone understood the importance of our mission.