Aptitudes are a measure of how fast you can advance in a skill. Every species has separate aptitude rating for each skill, and the higher your aptitude, the less experience you will need to advance the skill in question. Aptitudes are rated from -5 to 9, with 0 being average; the higher over that you are for your main skills, the better. Aptitudes are determined by your species, though manuals increase an aptitude by +4 for a finite amount of training.
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, or check each each species' individual pages on the wiki.
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|
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 means that every interval of +4 aptitude means half/twice as much experience. A +1 aptitude requires half as much experience as a -3 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.
|Maces & Flails||-0.560||Shields||-0.200||Summonings||-0.400|
|Ranged Weapons||-0.640||Air Magic||-0.760|
- Prior to 0.7.0, aptitudes were expressed in numbers smaller or larger than 100 (average), as in the Skill Points table above.