Your aptitudes are determined solely by your species.
As a result, certain species are biased or specialized towards certain backgrounds. For example, Deep Elves perform much better at magical backgrounds than purely melee-oriented ones, while the reverse is true for Minotaurs. One of the keys to winning the game is paying attention to your species' aptitudes, and choosing a playstyle that takes the most advantage of them. Luckily, the character select screen helps you with that, as backgrounds that your species has good aptitudes for are highlighted as "recommended".
?% in-game to see the big table of races and aptitudes.
Aptitude / XP formula
If you're interested in the actual math, an aptitude of
n means that you need
2^(-n/4) times as much exp to advance as a character with an aptitude of 0. Notably, this formula means that any pair of aptitudes whose difference is 4 differ by a factor of 2 in the skill points required to advance them: it takes twice as many skill points to advance a -3 aptitude as it does a +1 aptitude, for example.
Example: A Vampire has a +4 aptitude in Hexes, while a human has an aptitude of 0 in that skill. If it would take a Human 100 experience points to advance a level in Hexes, it will take the Vampire
2^(-4/4) -> 2^(-1) -> 0.5 times as much experience, or 50 experience points to advance a level.
The table below shows the relative experience needed to go up a skill level, assuming that with the aptitude of 0, 100 experience is needed. For example an aptitude of -4 means 200 experience is required to go up a skill level, or twice as much as aptitude 0.
|-4||(200)||very poor aptitude|
|-1||(119)||slightly disfavoured aptitude|
|+1||(84)||slightly favoured aptitude|
|+3||(59)||very strong aptitude|
Occasionally an aptitude may be listed as "NA". This means that the species is unable to learn or use that skill entirely, one of the most notable examples being Felids with weapon skills.
Prior to 0.7.0, aptitudes were expressed in numbers smaller or larger than 100 (average), as in the Skill Points table above.