www.mindfood.com/article/deepening-dependence/I found this article very interesting.
The only qualification I would add for now is, I looked up the studies on people undergoing pain and getting their brain scanned, and they only tested it with heterosexual partners, with the female partner receiving the shock and the male partner holding her hand.
Every article I have seen refers to it as "couples", and that bothers me that they are generalizing the results that way. It doesn't mean the effect is not there for a male getting a shock with his female partner holding her hand, it means that hasn't been studied.