I got 12 personality and aptitude at the start.
I did his knife thing a bunch of times.
Then I finished mission 1.
Then I did his knife thing a bunch more times.
When he lets you join, you are unable to do the knife thing - when you ask him what he wants, he tells you he wants a job, and the knife-throwing is inaccessible.
So, since I did the knife thing several times in several different instances, Niven must have not let me join, right?
So something was wrong...