They aren't really domesticated. Native South Americans probably have occasionally made pets from them for thousands of years and modern people have continued that practice. You can make pets out of wild animals but typically they don't behave in a very domestic way. Some people have luck with monkeys but they can be a handful.

I'm doing a paper for Biology and one of the questions is when my animal(Squirrel Monkey) was Domesticated. I have looked fr a few hours and cant find any info about it. Can someone please help me out and maybe a Link or two would be Appreciated. Thank you. :D