In the 4th century BC, the Greek philosopher Plato argued that love directs the bonds of human society.
In modern psychology, attachment of one individual to another individual is known as affectional bond. John Bowlby in his work on attachment theory elaborated the five criteria for affectional bonds, and a defined a sixth criterion for attachment bonds. Developmental psychologist Mary Ainsworth, described the characteristic of affectional bonds as follows:
1. Persistent, not transitory.
2. Involving a particular person who is not interchangeable with anyone else.
3. Involving a relationship that is emotionally significant.
4. Involving a desire to maintain proximity or contact with the person with whom the individual has an affectional tie.
5. Involving a feeling of sadness or distress at involuntary separation from the person.
An attachment bond has an additional criterion, that is, the person seeks security and comfort in the relationship.