Adult-onset amnesic patients had made very few new close friends since onset. Being unable to discuss recent shared events or return to previous conversations appears to block the bonding process.
How it was done
A study of adults with adult-onset amnesia, examining the friendships they had formed in the decades since onset.
What it means
This is as close to a natural experiment as this area gets. Remove the ability to retain episodic detail about another person, hold most other capacities steady, and new closeness largely stops forming.
It identifies which link in the chain is doing the work. Warmth, interest and social skill are not sufficient on their own; the ability to carry something from one conversation into the next appears to be load-bearing.
It also explains why the second conversation matters so much more than the first. The second one is where retained detail either shows up or does not.
