He observed various tricks followed by unusual gestures and even took into account the movement of every single facial muscle. After analysing in depth study of human facial muscles accompanied by expressions with respect to emotions he presented two models of facial expressions, "Neuro Cultural Model" and "Social Ecology Model". Neuro Cultural Model also approved and researched by Ekman, states that human being's facial expressions are influenced by cultural as well as biological motives, so the attitude is subjected to change according to one's culture, gender and social status. Social Ecology Model serves as an alternate to cultural model.
Although Darwin was not the first one to research on the emotional expressions of human beings. Before Darwin the topic was researched by Charles Bells who believed that facial expressions including smiles, frowns, laughs, sighs, beams, grimace, disgust and anger stand for humans as a natural language to communicate with each other, infact he possessed the view that facial expressions are the main source by which one soul communicates with another.
Darwin denied the theological foundation for emotional expression that Bell presented and analysed after making comparisons of facial movements in children, adults, the insane, as well as in animals like monkeys, dogs and cats done with the aid of photography and sketches. He proved by such comparisons several similarities across ages, sexes and mental capacities. (Hodge & Radick, 2003, p. 110) Darwin also analysed the expressions among ancestors and proved that human facial expressions have nothing to do with the communication or souls (Bells' Theory). Its is a certain mental state which is accompanied by actions that brings relief or gratification, then those actions thereafter accompanies the mental state for example, the turning away and the wrinkled nose of disgust, elicited originally by the sight of some repulsive object, might again be displayed due to the feeling alone. Darwin called this the 'principle of serviceable associated habits' and used it to explain various facial gestures like frowning, dejection, smiling and so on. (2003, p. 110)
While analysing different states and gestures, Darwin realised the importance of facial expressions in cultural as well as cross cultural studies and proposed that emotional facial expression plays a major role concerning cooperation among members of a society because it allows an exchange of information about affective reactions and behavioural tendencies. In the field of human interaction, the importance of facial expression can be best proved by the example that concerns interaction between mothers and their infants. (Scherer, 1988, p. 162)
Besides Neuro cultural model, Darwin also emphasised on cross cultural studies in context with facial expression of emotion, but most of the research is done by Ekman who described the human face as an important source of information in social interaction. Ekman (1978) describes the face as a multimessage, multisignal semiotic system. (1988, p. 162) Ekman along with Friesen continued the work of Darwin on 'muscle movement', and after analysing several studies, which include six pictures of posed, and