According to the matching hypothesis, people of approximately the same level of physical attractiveness will match up with each other. Because people often adhere to social norms, they are more likely to have relationships with people of similar age, race, religion, and socioeconomic class. Due to the reciprocity effect, displays of warmth and affection or displays of coldness and contempt towards a person are likely to elicit the same response from that person. Robert Sternberg developed the triangle theory of love, in which he proposed eight main types of love. People have an ideal triangle, the balance of components that they think is ideal, and a perceived triangle, the balance of components that they perceive as existing in their relationship.
Relationships are most successful when the ideal and perceived triangles of both partners all match.
Similarly, John Lee developed the color theory of love, in which he proposed six main types of love. John Bowlby developed attachment theory, which is used to explain the bond between infants and their caregivers. Secure attachment in a child arises from consistent and ample love, support, and proximal availability from the parent. Secure children are sociable and have stable reactions to separation and reunion with parents. Avoidant attachment arises from consistent coldness and unavailability from the parent.