Another of the newest latest elements to the game are personalities...when you catch a Pokémon they all have a random personality, and most of them have a slight effect on your stats...when it makes the stats go up, it's by a small factor of between 1 and 1.15, so it is barely noticiable, and if it goes down it would be between 0.85 and 1, hardly noticiable so it stays within the set Pokémon's Perameters. You can find out the personality by looking at the bottom box on the 1st page of the Status Screen. Anyway, here's a table naming them and showing what the raise and lower:

Natures Name	Raises	Lowers
Hardy	None	None
Lonely	Attack	Defense
Brave	Attack	Speed
Adamant	Attack	Special Attack
Naughty	Attack	Special Defense
Bold	Defense	Attack
Docile	None	None
Relaxed	Defense	Speed
Impish	Defense	Special Attack
Lax	Defense	Special Defense
Timid	Speed	Attack
Hasty	Speed	Defense
Jolly	Speed	Special Attack
Naïve	Speed	Special Defense
Modest	Special Attack	Attack
Mild	Special Attack	Defense
Quiet	Special Attack	Speed
Bashful	None	None
Rash	Special Attack	Special Defense
Calm	Special Defense	Attack
Gentle	Special Defense	Defense
Sassy	Special Defense	Speed
Careful	Special Defense	Special Attack
Quirky	None	None