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Biological psychology makes the undeniable point that psychological processes rest upon a biological substrate. Psychological phenomena occur within the context of our physical embodiment, so biological structures and processes clearly play a role in behaviour and cognition. (This is clearly illustrated by alterations in behaviour and emotional state produced by ingestion of drugs, exercise, brain damage etc.). Biological psychology therefore explores the potential roles biology can play in attempts at psychological explanation.\nThere are two main types of psychological explanation coming from biological perspectives. Causal explanations focus on the immediate precursors or causes (e.g. physiological processes) of a behaviour or characteristic; essentially, how a particular behaviour has occurred. These will be explored further in the rest of this section. Functional explanations, in contrast, look at why a particular behaviour or characteristic has evolved, i.e. the possible adaptive value of the behaviour seen within the context of neo-Darwinian evolutionary theory. This is examined in the section on evolutionary psychology. A central debate within psychology is the relative influence of social and biological factors. Relatively few psychologists would take the position that biology alone determines psychology, i.e. can fully explain all psychological phenomena. Psychological phenomena are usually seen by many biological psychologists as the result of a complex interdependence between biological and social processes. This is fully in line with the emphasis in modern genetics on gene-environment interaction, rather than seeing genes alone as a causal influence. For example, the same biological influences can have different effects in different social contexts. Social context can in turn affect biology (such as stress affecting the functioning of the heart).

The central focus of biological psychology looks at how the workings of the central nervous system (which includes the brain) affects behaviour and cognition. There may also be an emphasis on lessons that can be learned from the study of non-human nervous systems, looking at commonalities between different animals as well as those aspects in which humans are quite distinct from non-human animals. Although differences between people are examined in biological terms (e.g. in terms of brain damage or reactions to drugs), it is not concerned with what makes each human being unique. It is more concerned with documenting biological universals than with making individuals themselves the unit of analysis, offering a clear distinction here from perspectives like humanistic psychology. A key distinction between biological psychologists and psychologists from other traditions lies in the kinds of questions they ask. For example, a biological psychologist might look at depression in terms of neurotransmitter levels, or a particular genetic inheritance. A social psychologist might examine the depressed person's social networks and relationships. A more sociologically-influenced psychologist from a feminist background might in turn see the problem in terms not of the individual or their immediate social surroundings, but as a consequence of wider societal structures, e.g. oppressive gender relations within marriage as an institution. These different 'diagnoses' would lead to quite different courses of action in these three cases: respectively, recommending a course of anti-depressants; suggesting counselling; engaging in wider socio-political transformation. A psychologist taking a holistic viewpoint might conceivably regard all three as potentially useful actions to take.

In terms of methodology, biological psychology draws on a wide range of methods developed in disciplines such as neurophysiology, physics and chemistry, often involving study of the brain. For example, recording the electric activity of single neurons to see how they react for example to light stimulation, or studying the effects of stimulating neurons electrically. Biochemical analyses can also be used to monitor the activity of chemical neurotransmitters in the brain. Some relatively recent techniques include brain imaging techniques (e.g. positron emission tomography, or PET) and making use of data from the recent decoding of the human genome. In addition to these specialized techniques, biological psychologists also use the experimental method to compare the performance of different groups of people (e.g. with or without brain damage) on various psychological tasks. All the methods discussed above predominantly use an outsider viewpoint – although some recent brain imaging techniques do make use of insider accounts since researchers ask people questions about their experiences as they record brain activity, looking for correlations between the two.