Due to the high gravity, the human inhabitants of Jinx have short, squat bodies with great strength (and a predisposition to heart failure). In The Borderland of Sol, Julian Forward says:

"We bred for strength, we Jinxians. At what cost to other factors? Our lives are short, even with the aid of boosterspice. Longer if we can live outside Jinx's gravity."

They also tend to have very dark skin because of the intense sunlight of Sirius. Jinxian architecture is similarly shaped, with short, squat buildings so as to not collapse under their own weight. Jinxians are also known for their proclivity toward puns, such as the name of their capital, Sirius Mater: "mater" is Latin form "mother", but the phrase as a whole sounds like "serious matter".

Sources and references: World of Ptavvs, The Ethics of Madness, At the Core, Flatlander, Grendel, The Borderland of Sol,

Other sites: Known Space Concordance, Larry Niven Wikia, Wikipedia,