▁conducted ▁empir ical ▁studies ▁into ▁development al ▁changes ▁in ▁person ality ▁trait ▁construct s ▁across ▁the ▁lif es pan . ▁ ▁In ▁the ▁cogn itive ▁ab ilities ▁domain , ▁C att ell ▁research ed ▁a ▁wide ▁range ▁of ▁ab ilities , ▁but ▁is ▁best ▁known ▁for ▁the ▁distinction ▁between ▁fluid ▁and ▁cry st all ized ▁intelligence . ▁He ▁distinguished ▁between ▁the ▁abstract , ▁adapt ive , ▁bi olog ically - infl uen ced ▁cogn itive ▁ab ilities ▁that ▁he ▁called ▁" fl uid ▁intelligence " ▁and ▁the ▁applied , ▁experience - based ▁and ▁learning - en hanced ▁ability ▁that ▁he ▁called ▁" c ry st all ized ▁intelligence ." ▁ ▁Thus , ▁for ▁example , ▁a ▁mechan ic ▁who ▁has ▁worked ▁on ▁air plane ▁engines ▁for ▁ 3 0 ▁years ▁might ▁have ▁a ▁huge ▁amount ▁of ▁" c ry st all ized " ▁knowledge ▁about ▁the ▁work ings ▁of ▁these ▁engines , ▁while ▁a ▁new ▁young ▁engineer ▁with ▁more ▁" fl uid ▁intelligence " ▁might ▁focus ▁more ▁on ▁the ▁theory ▁of ▁engine ▁function ing , ▁these ▁two ▁types ▁of ▁ab ilities ▁might ▁complement ▁each ▁other ▁and ▁work ▁together ▁toward ▁ach ieving ▁a ▁goal . ▁ ▁As ▁a ▁foundation ▁for ▁this ▁distinction , ▁C att ell ▁developed ▁the ▁invest ment - model ▁of ▁ability , ▁arg uing ▁that ▁cry st all ized ▁ability ▁emer ged ▁from ▁the ▁invest ment ▁of ▁fluid ▁ability ▁in ▁a ▁particular ▁topic ▁of ▁knowledge . ▁He ▁contributed ▁to ▁cogn itive ▁ep ide mi ology ▁with ▁his ▁theory ▁that ▁cry st