…is from Lecture 6 “Rational Behavior” (page 25-26) of Gary Becker’s excellent book Economic Theory (original emphasis):
One basic query is: What is meant by rational behavior? Consider first what is not meant. Certainly not that people are necessarily selfish, “economic men” solely concerned with their own well-being. This would rule out charity and love for children, spouses, relatives, or anyone else, and a model of rational behavior could not be so grossly inconsistant with actual behavior and still be useful. A viable definition of rationality must not exclude charity and love; indeed, consistent family behavior probably requires love between family members.
Also, rationality should not imply that each household’s decisions are necessarily independent of those made by others. Different households are linked utltimately by a common cultural inheritance and background, and they may also be linked in a more proximate way. If household j increases its consumption of X, household i might be led to change its consumption of X. Such interdependicies commonly occur, and should be consistent with our model of rational behavior.
The essence of this model of rational behavior is contained in just two assumptions: each consumer has an ordered set of preferences, and he chooses the most preferred position avaliable to him.