Here is from wiki:

Tier I: You're not stupid! Just... different. Randomly receive 3x XP from any action, and the lower your Intelligence, the greater the chance.

Tier II: You now randomly receive 5x XP from any action. The lower your Intelligence, the greater the chance

Tier III: Randomly receiving bonus XP from any action may trigger 3x XP for all kills for a short period of time. The lower your Intelligence, the greater the chance.

Ps! it's not tier, but ranks...

At very high level, about 100, the hostiles gives very less xp when you kill them, but you get very high xp when you report in radiant quests (Int + perk + companions releationship). I realized that rank I, II and III doesn't stack. If you choose III, you cannot get anything from II. I was testing this today. I have rank III. Got the perk enabled, but I only got about 900 xp fighting some Super Mutants. When I had rank II, I got 5x 550xp by report in one spesific quest to the Institute. It happens more or less everytime when I report in all 5 radiant quests at the same time. So, I need to decide if I should nerfe myself and use console command to remove the rank III back to rank II. Low level character, player, may choose rank III.

Cheers,

-Klevs