For high INT
Flat bonuses to all skills above level 1:
INT 14 = +1
INT 16 = +2
INT 18 = +3
INT 20 = +4
Bonuses to mutagen creation results. Increase likelihood of positive mutation.
INT 14 = +10% positive / -10 chance of negative mutation
INT 16 = +15% positive / -15
INT 18 = +20% positive / -20
INT 20 = (with cooking 10) complete control: create targeted mutagens
That is to say, mutagens should record the intelligence and skill level of their creator at the time of creation. The game might need imaginary scientists. These scientists would have generated names and stats, and the results of their work (mutagens / serums) could be found all over the world. Or just in a particular lab. Whether these scientists are dead or alive, is a separate matter. Ideally you'd only use mutagens created by someone with two university degrees and 20 years of experience in biochemistry (for example). That is to say, imaginary scientists could have A) education level and B) work experience measured in years.
For low INT, cut learning speed:
INT 8 = -15%
INT 6 = -20%
INT 4 = -30%
Also set caps for certain skills:
INT 12: max computers = 10
INT 10: max computers = 8
INT 8: max computers = 5
INT 6: max computers = 4
INT 4: max computers = 3
Consider similar caps/tables for each skill individually.
But that is to say you should still be able to able to start with INT 4 and computers 8. You'd be a savant of sorts. You just wouldn't be able to learn more. Whether or not skill rust should affect this, or to what degree, is another matter. Of course in the long run it'd be smarter (easier) to start with INT 8 and computers 4 IF the choice is given. But something like computers 8 is pretty juicy in a lab start scenario. Also this affects random character starts and RPing.
Problems or challenges may arise when one uses INT boosting drugs. Drug effects should last longer anyway (for reading), and they should have longer lasting side effects, or more insidious side effects that don't manifest until after sufficient use, but when they do, it's bad, and it lasts. After all, it should be possible to compensate low INT (or boost moderate INT) with certain drugs, to a degree, and with a price.
Of course books could have hard INT requirements, on top of current soft requirements. Some books could be more accessible than others. Also, consider something like this:
This book requires fabrication 3 OR INT 14.
Higher intelligences could also extract more enjoyment out of challenging reading. But that could be a trait as well, recommended only for high INT characters. I mean, just because you have high INT, doesn't necessarily mean you enjoy reading. In fact, reading could be really annoying pastime.
Could higher INT characters gravitate towards certain mutations? Such as "Easily Distracted" or "Focused" or "Easily Annoyed" or "Severe Headaches" or "Zen Mind" or...
Should we have traits that are meant only for high INT or low INT characters? Then again, if we go down that road, should there be traits for high STR or PER characters?
High INT characters could also suffer mentally (negative morale) in the presence of lower INT NPCs, but also enjoy more if a given NPC also has high INT. That is to say two INT 18s enjoy each other's company more than two INT 12s. And between high INT character and low INT NPC, the greater the INT difference, the stronger the negative morale effect.
Should Cata have aspects of different languages? What's the language base of New England? 99% English or more? Higher INT characters might be able to pick up foreign languages faster, and have more alternative routes to knowledge/objectives at their disposal, assuming there is any foreign language literature around. German cook books? Black box transcripts in French? Survivor notes in Spanish? A character could also start out as multilingual. Higher language skill = the greater portion of "foreign language content" appears in English. For example:
(this message is in Spanish, your Spanish is 3 (38%))
Bro, don't go ____. We can ____ but then ____. ____ and pick up the ____ . ____ is 3872. -Hector
Something to consider is that it's also possible that what's keeping INT back, is the lack of all the other content. And some of us know the game pretty well, and it becomes increasingly harder to bring (more) meaning to INT. :/