Psychologists try to understand the working of the human mind (thoughts, emotions, principles, experience) and how this affects one's behaviour. It is a separate discipline from Sociology but does assists sociologists in explaining some sociological processes. Sociology can also help in explaining some of the aspects of Psychology, for example: how one's social environment (sociology) influenced his principles and beliefs (psychology).
Political Science can be regarded as a branch of Sociology; dealing mainly with the political aspects of Sociology. Sociology covers politics in society but Political Science is more in-depth.
Economics can be considered to be a highly complex branch of Sociology making it difficult to identify a relationship between the two without having knowledge of both disciplines. Economics focuses on wealth, trade, consumption behaviour patterns, et cetera and even involve some complex models rooted in the discipline of Mathematics.
Anthropology is oftentimes associated with the study of simpler societies while sociologists study more complex societies. Also, it is said that while sociology tends to focus on social institution, anthropology is more holistic in its approach to society. As the two disciplines develop, similarities are being seen and the lines between the two disciplines are becoming more blurred.
Demography is highly statistical and provides the numbers (statistics) for the proving of some sociological theories. Dealing mainly with populations, demographers come up with their statistical data by studying population behaviour and...