I believe the answer is: Role confusion
Identity vs role confusion occurs when and individual is at the period of time when he/she is trying to find what is the purpose of their life. People commonly reach this stage when we are transitioning from childhood (when our roles are fully determined by our parents) to adolescent.
This is an example of exchange theory.
In sociology, exchange theory has to do with maximizing benefits while also minimizing costs. When applied to Bill and Cathy's situation, they are taking advantage of each other's skills in order to minimize costs - Bill can cook and Cathy can't (so they can prepare food on their own) and Cathy can repair stuff and Bill can't (so they don't have to get a handyman).
The answer is "Heritability".
Heritability reflects the way that all people in any types of living things contrast from multiple points of view among each other. The variety (~differences) among people inside a species relies upon both genetic and ecological contrasts. For instance, individuals differ in height.
