I understand why my fingers wrinkle when swimming. But can you tell me why my wife’s fingers do not. Not only her but every member of her family-cousin,neices,Aunt. My wife is Cambodian. I think I may have seen this in Japan also. Japanese people also do not get wrinkled fingers when swimming.
Beat me by a minute, MBS. Gonna post my link anyway, just cuz I took the trouble
http://www.straightdope.com/classics/a2_317a.html
Basically, the skin is absorbing water, and thicker skin on the fingers and toes expands with nowhere to go.
Dunno why the OP’s Inlaws seems immune to the effect, though.
I have read that answer and other web answers.
The question I have is why this does not happen to Cambodian people. Came back from pool today where I spoke to Cambodian friends of my wife’s Aunt. Asked them to show me their fingers. No effect. They were in the spa for 45 minutes.
To repeat -I understand why it happens to me. But why do the fingers of Cambodians not get wrinkled when swimming?
<tongue in cheek> …maybe they’re water-repellent?
Seriously. I swear I’ve answered this question (about why fingers and toes get all pruney) dozens of times for my kids, and all without reading The Master’s answer even once. 
Then I typically branch into the surface of the stomach and intestines, and the convolutions of the human brain, and how they’re all kind of the same, and you get more surface area in the same space that way.
My kids are never impressed.