In his monumental book, The Mind of Primitive Man, he set forth his conviction that there is no fundamental difference between the mind of primitive man and that of civilized man, and that whatever differences exist between the achievements of different races could be accounted for on historical and cultural grounds rather than in terms of innate biological differences.
Lewis has also established the role of psychology in understanding the ethnology of primitive peoples. Social psychology, however, is the closest kin of social anthropology.
It deals with the mass behaviour of the primitive peoples. If, for example, the tribals are influenced by the Devi movement in south Gujarat, it is a clear case of the relationship of social psychology and social anthropology. However, general psychology interprets the behaviour of man as his individual manifestation.
Regarding the relationship of psychology with social anthropology, it can be said that there are two different realms dealt with by the two disciplines. Psychology is concerned with the behaviour of man and social anthropology focuses on interrelationship among different institutions of the society.
There is a basic contradiction regarding the approach adopted by the two disciplines. Psychologists begin by taking the tribal culture for granted, as if it were uniform and universal. With this assumption they study psychic behaviour in a tribe. On the other hand, the same approach is taken by social anthropology.
It tends to take human nature for granted, as if it were uniform and studies the diverse cultures of the tribe. In reality, in technical language, we have two variables, viz., mind and culture, and each science, that is, psychology and social anthropology, assumes that it can go ahead by treating the other variable as if it were constant.
But researches now undertaken in both the fields have made it clear that such constancy is not actual. But, to deal with two highly complex variables is difficult; and as for specific findings, only beginnings have as yet been made.
W.H.R. Rivers, who conducted his studies among the Todas of South India, has dealt with the relationship of psychology and ethnology or social anthropology. He has argued for the independent science of social anthropology, though ultimately the discipline is related to human behaviour because, as he observes:
The final aim of the study of society is the explanation of social behaviour in terms psychology.
To simplify the relations between psychology and social anthropology, it could be stated, that the study of interrelationship in a primitive society owes much to the psychological conditioning of the people in that society.
This can be illustrated by referring to the situation of Indian tribals. The tribals, historically, have been exploited by the moneylenders and traders. I.P. Desai notes that when there is a meeting of the tribals of Bardoli taluka, they simply prohibit the entry of non-tribals.
They consider caste Hindus as their enemies. This psychological mindset determines their behaviour. Therefore, in any study of primitive society, the psychology of the primitives constitutes a relevant point.
This also explains the relationship between social anthropology and psychology; the former is concerned with a variable of culture and the latter with the mind.