Science Fiction Trivia Time!

Yes, it's trivia time again for all us fans of science fiction.  This time were hitting the books.  I'm going back to an old favorite of mine, Larry Niven's Ringworld novel.  Better break it out if you haven't read it...

Ok.  Today's question is:  In Niven's Ringworld, as well some of his other Known Space books, he incorporated an idea that a certain genetic trait could be favored by selective breeding.  What is the trait?