McConaughey just stumbled into the most studied result in game theory and presented it as a hunch about being a good neighbor.
In 1980, a Michigan political scientist named Robert Axelrod ran a tournament. He invited mathematicians and economists to submit strategies for a game where you repeatedly choose to cooperate with someone or screw them over. Fourteen entries, some hundreds of lines of code. The one that won was four lines: cooperate on the first move, then copy whatever the other person just did.
He ran it again with 62 entries, everyone knowing it had already won. Nobody could beat it.
The traits that made it win: never defect first, punish defection, forgive fast, stay predictable. "Slowed down, let her in" is line one of that program.
Here is the part Matthew got right without knowing the math. The reason cooperating first wins, and doesn't just feel nice, comes down to what theorists call the shadow of the future. Be generous to a stranger you'll never see again and you eat the cost for nothing. Be generous to someone you'll keep running into and the move pays itself back across every interaction left to come.
He thought he was playing a one-shot game with an anonymous driver on a highway. He was playing a repeated game with a neighbor. Same road every day, same faces, decades in front of both of them.
That is why the favor returned in 15 minutes, and why it keeps returning. A highway full of strangers looks like the one place generosity gets wasted. In a small enough world there are no strangers, only people who haven't repaid you yet.
Matthew McConaughey reveals how letting one car merge in traffic gave him a neighbor watching his house for life
"Two-lane highway, traffic jam, moving 5 mph. A lady was waiting to get in. Everyone wants to get forward as quick as possible. I slowed down, let her in"
"15 minutes later, I'm right behind her. As I approach my house, she pulls in the driveway right before me. I get out - it's my neighbor"
"I didn't know it was my neighbor. But I got somebody watching over my house from now on because I let her in. Did I do it hoping to get a neighbor on my side? No. Did I get an army on my side? Yeah"