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Factors Affecting Behaviour, Emotion and Thought

Psychology (Year 12) - Biological Influences

Jessica Pratt

Many factors can impact the behaviour, emotion and thoughts that an individual possesses. These can include heredity (the role of genetics), hormones, and psychoactive drugs.


The passing on of characteristics genetically between generations. It is generally studied through twins, where identical twins develop through a singular fertilised ovum, whereas fraternal twins develop from separate ovum that were fertilised at the same time.

Schizophrenia is a mental disorder resulting in individuals experiencing an altered sense of reality such as hallucinations and delusions. It has been found that there is a strong genetic component associated with its development. When one identical twin has the disorder, there is a 50 percent chance of development in the other. When a child has two parents with schizophrenia, there is a 46 percent chance of development. An individual has a one percent chance of development when there is no family history.

Epigenetics is the study of changes in organisms caused by the modification of gene expression, rather than alterations made to the genetic code.

Diathesis-stress Model

Works to explain how both genetics and the environment have a role in the development of a mental illness. People are born with a genetic predisposition to a mental illness, but not all will develop it.

There are two components to the model:

  1. diathesis is the vulnerability of an individual to a mental illness.

  2. stressors are the event that disrupts equilibrium.


Hormones are chemical messengers that are produced by endocrine glands and are transported via the bloodstream. They are slower to act but are longer lasting than nerve impulses.

The hormones of interest include:

  • adrenaline is produced by the adrenal medulla to stimulate the “fight-or-flight” response in order to cope with short-term stress and to increase blood glucose levels.

  • endorphins are neurotransmitters that trigger positive emotions and decrease pain perception during strenuous exercise, pain and sex, as they induce pleasure and euphoria.

  • glucocorticoids, including cortisol, are produced by the adrenal cortex during the long-term stress response, whilst also functioning to reduce inflammation, and regulate metabolism.

  • sex hormones such as oestrogen and testosterone stimulate the sexual characteristics and sex drive.

Psychoactive Drugs

A drug is a substance that can influence the body to function abnormally. In addition to this, a psychoactive drug is a chemical substance that acts upon the central nervous system to alter brain function, leading to changes in in perception, mood, consciousness and behaviour.

There are numerous classifications of psychoactive drugs, including:

  • a stimulant increases the activity of the central nervous system.

  • depressants work to decrease the activity of the central nervous system.

  • narcotics work to reduce pain, alter mood and induce sleep.

  • an analgesic reduces or eliminates the pain impulse.

  • a tranquiliser is a depressant that relaxes muscles, reduces anxiety and induces sleep.

  • hallucinogens induce hallucinations and alter sensory perceptions.

Effects of Psychoactive Drugs

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