Actually, the surprise here was how much the environment plays into maintaining (or losing) cognition over time. The intelligence you’re born with is mainly determined by genes (as the article notes, mainly from excellent studies looking at identical twins reared apart), so the findings here were perhaps unexpected by some researchers in this field.
I’m guessing part of the environmental would include how much ‘exercise’ an individual gives their brain . It’s like a muscle use it or lose it.
My grandmother’s common sense could have predicted this. If you have the ability to understand and learn math, but you never use it throughout your life, you won’t remember how to do algebraic equations when you are 90.
Those famous “Twins Studies” from the University of Minnesota were conducted by Thomas Bouchard, formerly my neighbor in Berkeley when he and my husband were both students at UC. I used to babysit his daughter, Elizabeth, daily to earn extra money. I wonder how many IQ points I shaved off of that child by being exposed to me every day! LOL