Well... my understanding is that over-thinking is primarily a product of anxiety. So, assuming that is the case, when asking if over-thinking is learned or something one is born with, you'd have to ask the same question about anxiety. I don't know as there is really any definitive proof with regard to whether anxiety is learned or is something one is born with. Perhaps it is a bit of both. Here are links 3 to articles from PsychCentral's archives regarding what causes anxiety, the link between anxiety & over-thinking or ruminating, & what one can do about it:

https://psychcentral.com/disorders/a...ety-disorders/

https://blogs.psychcentral.com/pract...evere-anxiety/

https://blogs.psychcentral.com/imper...nking-anxiety/

Also here's a link to an article on the Healthy Place website regarding the link between anxiety & over-thinking:

https://www.healthyplace.com/blogs/a...ng-everything/