Mutual, and generally equitable, exchange of benefits is known as social psychology

the number of senators, and representatives, the state has in congress.

intimacy, passion and commitment are the components of a interpersonal relationship emphasized in the triangular theory of love by robert sternberg.

the triangular theory of love says love can be comprehend as this componest form the vertices of a triangle.  

intimacy relates to feelings such as of closeness and connection;

passion relates the drives that brings people together, as physical attraction.

commitment relates to the being in a long-term relationship.

answer; /// charles darwin;

stable food supply,social structure, government,religion,culture/art,technology and writing/language;