The process of indentifying the benefits and costs of different alternatives by examining the incremental effect on total revenue and total cost causes by a very small (just one unit) change in the output or input of each alternative. Marginal analysis supports decision-making based on marginal or incremental changes to resources instead of one based on total or averages.
I believe the answer is: indirect benefit
Indirect benefit refers to the type of benefit that we had from the action or existence of another people. In the example, the bond that formed between the two did not necessarily exist because of their actions/behaviour but because of their similar experience of taking care of other people.