Picture two neighbours and a fence. One never crosses it; the other crosses whenever it suits. The fence is in the same place for both, but for one it is a line and for the other a suggestion. A line is a rule about what is not done to someone, and it works only if both sides keep it.
I keep it for you→You keep it for me→The line holds
People call these lines boundaries: what may not be said to me, or done with my words. When one side keeps a line, it protects the other person. You are protected only if the other side keeps it too. The old rule, do not do to others what you would not want done to you, is half of it.
Samnever shares
vsvs
Joposts them
Sam never shares screenshots of chats. Jo posts them whenever a message is funny. Whose messages does Sam's habit protect?
Sam's own. A good habit protects the person who has it.
Not yet. Sam's habit decides what Sam does with Jo's messages. What happens to Sam's messages is decided by Jo, and Jo posts them.
Jo's. Sam keeps the line, so whatever Jo sends Sam stays in the chat.
Right. A line protects the person the other side keeps it for. Sam keeps it, and Jo is the one it protects. Sam's own messages depend on what Jo does, and Jo posts them.
Nobody's. Chats are never safe.
Not yet. Jo's messages are safe, because Sam keeps the line for them. A line kept by one side protects the other side.
Both keep the dealboth do well
vsvs
One betrayspays once
In 1979 and 1980 a political scientist, Robert Axelrod, ran a contest. People sent in computer programs to play a game many times. Each round, keep the deal or betray. Betraying a partner who keeps it pays most, once; both betraying pays both badly, less than both keeping it. Fourteen programs entered, then sixty-two. One program won both times.
Which kind of program do you guess won both of Axelrod's contests?
Pick one. Nothing is scored.
Keep the deal first→Copy their last move→Forgive at once
Keep the deal first, then copy what the other did last.Tit for tat, sent in by Anatol Rapoport, a few lines long. It never betrayed first, answered a betrayal with one betrayal, and went back to keeping the deal the moment the other did. It kept the line for anyone who kept it, and only for them.
Both keep the line3 points a round, the game's own
One crosses: the crosser5 points a round, the game's own
One crosses: the other0 points a round, the game's own
Both cross1 point a round, the game's own
Tit for tat is a fence with a memory. It keeps the line first, so a partner who keeps it is never punished. It answers a crossing with one crossing, the half the old rule leaves out, so breaking it gains nothing for long. And it forgets at once. It won because its partners did well too.
Set how often you keep the line against how often the other side does, and watch what each of you gets over many rounds. The points are the game's own; the mix is made up.
Your points1 a round
Their points1 a round
You keep it: Never · They keep it: Never
You keep the line: never. They keep it: never. You average 1 a round, they average 1.
Your points3 a round
Their points0.5 a round
You keep it: Never · They keep it: Half the time
You keep the line: never. They keep it: half the time. You average 3 a round, they average 0.5.
Your points5 a round
Their points0 a round
You keep it: Never · They keep it: Always
You keep the line: never. They keep it: always. You average 5 a round, they average 0. Five for you, nothing for them. A partner who keeps a line for someone who never does is paying for nothing, and tit for tat stops paying.
Your points0.5 a round
Their points3 a round
You keep it: Half the time · They keep it: Never
You keep the line: half the time. They keep it: never. You average 0.5 a round, they average 3.
Your points2.3 a round
Their points2.3 a round
You keep it: Half the time · They keep it: Half the time
You keep the line: half the time. They keep it: half the time. You average 2.3 a round, they average 2.3.
Your points4 a round
Their points1.5 a round
You keep it: Half the time · They keep it: Always
You keep the line: half the time. They keep it: always. You average 4 a round, they average 1.5.
Your points0 a round
Their points5 a round
You keep it: Always · They keep it: Never
You keep the line: always. They keep it: never. You average 0 a round, they average 5. You keep paying and get nothing. This is the cell tit for tat refuses to stay in.
Your points1.5 a round
Their points4 a round
You keep it: Always · They keep it: Half the time
You keep the line: always. They keep it: half the time. You average 1.5 a round, they average 4.
Your points3 a round
Their points3 a round
You keep it: Always · They keep it: Always
You keep the line: always. They keep it: always. You average 3 a round, they average 3. Three and three is the highest shared total.
Where do both sides do well at once?
Only where both keep the line always: three points each.
Right. Every other cell gives at least one side less. Betraying a partner who keeps the line pays five once, and the frames show what the partner gets: nothing. Tit for tat made sure nobody kept paying that.
Where you never keep it and they always do: five points.
Not yet. Five for you, zero for them, and a partner who keeps a line for someone who never does is paying for nothing. Tit for tat answered that cell with a crossing the next round.
Nowhere. It is a game where someone always loses.
Not yet. Both keep gives three and three. Both cross gives one and one. The shared cell is better for both than the fight.
Move both controls.
Each card is a line in a phone's world. Is it kept by both sides, or by only one?
One card at a time. Tap the pile it belongs to.
Card 1 of 5
Two friends who agree that nothing from the chat leaves the chat
A person who never clips other people's streams, whose own streams are clipped daily
A group where everyone asks before posting a photo of someone else
A commenter who gets polite replies and leaves cruel ones
Two neighbours who each keep the music down after ten
Two friends who agree that nothing from the chat leaves the chat → Both keep it
Not yet. Both. Each keeps it for the other, so each is protected by the other.
A person who never clips other people's streams, whose own streams are clipped daily → Only one keeps it
Not yet. Only one. The person keeps a line nobody keeps for them. For them it is a wish, and tit for tat would not stay there.
A group where everyone asks before posting a photo of someone else → Both keep it
Not yet. Both. The asking runs in every direction, which is what makes it a line and not a favour.
A commenter who gets polite replies and leaves cruel ones → Only one keeps it
Not yet. Only one, and the wrong way round. The others keep the line for the commenter, who crosses it, which is the five-and-nothing cell.
Two neighbours who each keep the music down after ten → Both keep it
Not yet. Both. Each gives up the same thing for the other, and each gets the same quiet back.
3Both keep it
vsvs
2Only one
Right. Three lines that hold, two that do not. A line that one side keeps is not a line. It is a gift to the other side, and tit for tat is the rule that stops it staying a gift.
Clips the rival
↓→
A week
↓→
Gets clipped
A streamer posts a clip of a rival's embarrassing moment. A week later the rival, and people who saw that clip, clip the streamer's own moment and spread it. Using this lesson, what happened to the line?
It was never a line. The streamer kept it for nobody, so nobody kept it for them.
Right. The streamer crossed first and the rival copied the move, and so did the people watching. One crossing invites one in return, which is the rule that won Axelrod's contest.
The rival broke the line first by being embarrassing.
Not yet. Being embarrassing crosses nothing. Posting the clip did, and the rule copies the posting, not the moment.
Bad luck. Clips happen to anyone.
Not yet. The prompt names who clipped back: the rival the streamer had clipped. That is the copying rule at work, not chance.
Lesson complete
A boundary holds only when both sides keep it; copying the other side's last move keeps it honest.