A social network is a social structure made up of nodes (representing individuals or organizations) which are connected (through "ties", also called "edges", "connections", or "links") by one or more types of interdependency (such as friendship, common interests or beliefs, sexual relations, or kinship). Social network analysis uses the lens of network theory to examine social relationships. Social network analysis as a field has become more prominent since the mid-20th century in determining the channels and effects of social influence. For example, Christakis and Fowler found that social networks transmit states and behaviors such as obesity, smoking, drinking and happiness.