It would be interesting to see a study controlling for the "sandwich years". My suspicion is that the sandwich years on top of 40+ hour/week employment causes a lot of cognitive degradation. But, 40+ hour/week employment without the sandwich years would see significantly less, if any, degradation.
From the article:
"McKenzie’s team is now looking into the driving factors behind their research such as the “sandwich years” when many adults have at least one person to look after, a child or an elderly parent, on top of working full-time."
This terminology stuck out because I'm smack in the middle of it right now. It's a regular conversation piece when my wife and I get together with other parents.