Introduction to Heston's study

Heston's (1966) research is an adoption study that explores the causes of schizophrenia, a severe mental disorder characterised by disruptions in thought processes, perceptions, and emotional responsiveness. This type of study compares individuals who were adopted away from their biological parents to separate genetic influences from environmental factors. By focusing on children of mothers with schizophrenia who were adopted at birth, the study aimed to determine whether genetics or family environment plays a larger role in developing the disorder.

Aims of the study

The primary goal was to challenge the idea that parents, particularly through creating a distorted family environment, could cause schizophrenia in their children. At the time, some theories suggested that a parent's behaviour might produce interpersonal distortions, with closer relationships leading to greater risks. Heston sought to test this by examining adopted children who had no contact with their biological mothers, allowing researchers to isolate potential genetic factors from environmental ones.

Participants in the study

The study involved two groups to enable comparison.

Study groups:

• Experimental group - This consisted of 47 participants (30 males and 17 females) born between 1915 and 1945 to mothers diagnosed with schizophrenia. These mothers were patients in an American psychiatric hospital, and the children were placed for adoption immediately after birth.
• Control group - This included 50 participants (33 males and 17 females) who were also adopted but born to mothers without schizophrenia. They were matched to the experimental group for factors like age and adoption circumstances to ensure fair comparisons.

Participants were selected based on strict criteria for the mothers in the experimental group, including a confirmed diagnosis of schizophrenia, evidence of behaviours consistent with the disorder, no other diseases, and complete separation of mother and child from birth.

Procedure of the study

Heston used a longitudinal approach, tracking the participants into adulthood to assess outcomes.

The procedure involved:

1. Identifying suitable participants from adoption records and hospital data, ensuring no contact between biological mothers and children.
2. Collecting detailed psychiatric evaluations for each participant, including assessments of mental health, intelligence, and psychosocial functioning.
3. Diagnosing schizophrenia only when there was unanimous agreement among two independent psychiatrists and Heston himself, with additional confirmation from a psychiatric hospital to ensure reliability.

Results of the study

The results highlighted significant differences between the groups, supporting a genetic component to schizophrenia.

Key findings:

• Schizophrenia diagnoses - In the experimental group, 5 participants (10.6%) were diagnosed with schizophrenia, compared to 0 in the control group.
• Other outcomes - The experimental group showed higher rates of psychosocial disability and IQ deficiencies.

Conclusions from the study

Heston concluded that the findings provide evidence for a genetic influence on schizophrenia, as the elevated rates in the experimental group occurred despite no exposure to their biological mothers' environments. However, not all participants in the experimental group developed the disorder, and some even showed exceptional talents not seen in the control group. This suggests that while genes play a role, other factors must also contribute to the development of schizophrenia.

Evaluation of the study

Strengths

• The adoption design effectively separates genetic and environmental factors, providing stronger evidence for heredity than family studies alone.
• Strict diagnostic criteria (requiring agreement from multiple experts) enhance the reliability of schizophrenia diagnoses.
• The longitudinal nature allows for observing long-term outcomes, adding depth to the findings.

Limitations

• The sample size is relatively small (97 participants total), which may limit generalisability to broader populations.
• Potential confounding variables, such as adoptive family environments or prenatal factors (e.g., maternal stress during pregnancy), were not fully controlled.
• The study focuses only on maternal inheritance, ignoring potential paternal genetic contributions.
• Ethical concerns arise from researching vulnerable populations, though the study used existing records to minimise harm.

Overall, Heston's work supports the diathesis-stress model, where genetic vulnerability (diathesis) interacts with environmental stressors to produce schizophrenia, influencing modern understandings of the disorder.