The 'Two-Hit' Hypothesis in Abnormal Psychology

Instructor: Emily Cummins
In this lesson, we'll talk about potential explanations for schizophrenia, a mental illness characterized by serious disruptions in thoughts and behaviors. We'll focus on a particular explanation for the disorder known as the two-hit hypothesis.


Schizophrenia is a mental disorder that causes serious disruptions in thoughts and behavior. Its onset is generally in late adolescence or early adulthood and involves a number of abnormal behaviors. Schizophrenia is characterized by positive and negative symptoms. Positive symptoms are things like delusions, hallucinations, strange body movements, and odd ways of thinking. Think of these as additions - things that healthy people do not have. Negative symptoms refer to issues with emotions and affect, which basically means how we display our emotions. This includes things like being expressionless, having a flat tone of voice, and difficulty finding enjoyment in everyday life.

Schizophrenia is not the most common mental disorder, but it impacts a number of people, and doctors and scientists still aren't really sure what causes it. One explanation for the occurrence of schizophrenia is called the two-hit hypothesis. Let's talk about what this means.

Two-Hit Hypothesis in Abnormal Psychology

Researchers who study schizophrenia have long tried to identify a cause for this disruptive illness. However, no single factor has been shown to be responsible for this disorder. While some research has shown that schizophrenia runs in families, there isn't always an identifiable genetic risk. Researchers also look to environmental factors, such as an early trauma, which are linked to developing schizophrenia.

The two-hit hypothesis states that a person is made vulnerable to schizophrenia by something early on in life, like a serious infection, and it is only later on when the person encounters an additional serious issue, such as psychological trauma, that schizophrenia is triggered. See how the language of two hits makes sense here?

Researchers think that some people might have some genetic susceptibility to the disorder but it really needs to be activated by what neurological researchers call an insult (but not like being called a mean name!). An insult is something like the psychological trauma we mentioned above. It's likely that genetic and environmental factors interact with one another and create conditions within the brain that can prime someone to develop schizophrenia.

Experiments and the Two-Hit Hypothesis

So why do researchers think the two-hit hypothesis might explain the development of schizophrenia? Let's talk about some experiments that have been done to provide evidence for this hypothesis. In one influential study with mice (yup, mice!) researchers exposed mice to a very loud noise which, as you might expect, startled them. This is a neurological test known as prepulse inhibition (PPI). But if the mouse heard a quiet noise before the loud noise, it tended to be less startled. However, one group of mice was infected with a virus and later on was given environmental stressors. When these mice were then given the PPI test and heard the second noise, they were just as startled as they would have been with the initial quieter sound. These mice were exposed to both sickness and stress (two hits).

So what does this mean? When researchers looked at the brains of these sick mice, they saw that some of the brain cells were actually changed. Specifically the immune cells in the brain, which are normally thin - these became much thicker in the mice exposed to both threat and infection. This is something we also see in humans with schizophrenia. Researchers found that the immune cells in the hippocampus, which is the part of our brain involved in regulating emotion, and in the prefrontal cortex, which is key to things like decision making, got much thicker. These are also the parts of the brain associated with schizophrenia.

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