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Essay: Discuss whether schizophrenia is inherited

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  • Published: 18 March 2024*
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Claim: Psychological disorders are inherited.

Rationale:

This research investigation will discuss whether schizophrenia is inherited.

DSM-V (Diagnostic and Statistical Manual of Mental Disorders IV) is the guide used by healthcare professionals for diagnosis of mental disorders, published by the American Psychiatric Association (American Psychiatric Association, 2013).

Psychological disorders are difficult to define in an overarching definition that covers all situations. The concept of a psychological disorder lacks a consistent operational definition to properly define the term. They have, however, been previously defined by using concepts such as distress, disadvantage, inflexibility, dyscontrol, irrationality, etiology, disability, and statistical deviation (Stein, D., Phillips, K., Bolton, D., Fulford, K., Sadler, J., & Kendler, K., 2010).
DSM-V considers these factors when defining psychological disorders: a behavioural or psychological syndrome or pattern that occurs in an individual; reflects an underlying psychobiological dysfunction; the consequences of which are clinically significant distress or disability; must not be merely an expected response to common stressors and losses or a culturally sanctioned response to a particular event; primarily a result of social deviance or conflicts with society (American Psychiatric Association, 2013).

Psychotic disorders are defined as a group of serious psychological illnesses that negatively impact a person’s life and function through abnormal thinking and perception (“Psychotic Disorders: MedlinePlus”, 2021). As a debilitating disorder to most affected individuals, schizophrenia is defined as a complex chronic mental health disorder characterised by an array of symptoms (Krishna R. Patel, 2021).

To be diagnosed with schizophrenia, two or more of the following must be present during a 1-month period, according to the DSM-V (American Psychiatric Association, 2013). At least one must be the first, second or third on the list: delusions, hallucinations, disorganized speech, grossly disorganized or catatonic behaviour, negative symptoms (American Psychiatric Association, 2013).

Heritability is defined in two ways. The statistical definition describes heritability as proportion of phenotypic variance attributable to genetic variance. The common definition defines heritability as the extent to which genetic individual differences contribute to individual differences in observed behaviour, or phenotypic individual differences (“Heritability: Introduction”, 2021).

This research investigation will be exploring the heritability of schizophrenia when one or both parents have the disorder in question, as there has yet been no conclusive evidence to support this.

Research question: Is schizophrenia inherited if the parents are afflicted with the disorder?

Evidence 1: Investigations into the Progeny of Schizophrenic Married Couples, written and conducted by German psychiatrist Ernst Rüdin-München Eugen Kahn, tests eight different families and heritability of schizophrenia in the case both parents are diagnosed with the disorder. To do this, a pedigree was created to show which children developed schizophrenia from the tested families (Kahn, 1923, p. 119-121).

Figure 1: Pedigree charts of the eight families tested for schizophrenia (Kahn & Rüdin, 1923).

All children in the first four families, when they reached an adult age, were schizophrenics according to clinical findings. However, it should not go unmentioned that the third child (Elisabeth Hell) of the Winzert couple and the fifth child (Gottfried Kreser) of the Kreser couple were diagnosed with schizophrenia from a purely clinical point of view, though there was doubt within their pronounced schizoid peculiarities (Kahn & Rüdin, 1923).

This shows that while there is not a 100% chance of a child inheriting schizophrenia, it increases their chances compared to those born to parents without schizophrenia. However, this does not prove that schizophrenia is inherently heritable.

Family 8 shows that despite both parents having schizophrenia, none of their four children developed any form of schizophrenia. This shows that this family is an anomaly, and that heritability of schizophrenia is not entirely reliable on genetics.

Limitations to this evidence include the fact two of the participants tested were given a diagnosis of schizophrenia, despite it not being entirely clear whether they had the disorder or not. Another limitation is the way the data was presented, shown in an atypical pedigree diagram with no decoding key, meaning the reading of the evidence could be incorrect. Similarly, only 8 families were tested, meaning there was less variance in the data, providing less of a range of data to be studied.

Evidence 2

In a meta-analysis – a statistical analysis that combines or integrates results of several independent clinical traits considered by the analyst to be “combinable” (Egger & Smith, 1997) – conducted by Patrick Sullivan, et al., 12 of 14 studies were chosen to be analysed. The twelve met the criteria that they had the consistent view of schizophrenia as a complex trait resulting from genetic and environmental etiological influences (Sullivan et al., 2003).

Figure 2: Variance component estimates for 12 twin studies of schizophrenia and the meta-analytic summary (Sullivan, P., Kendler, K., & Neale, M., 2003).
A test for homogeneity was strongly rejected with a P value of <.001. When comparing the methodologically superior studies of 27-29 to the methodologically inferior studies of 19-21, 23-26, and 30, there were similar point estimates for additive genetic effects (77% vs 78%) and common environmental effects (17% vs 14%). There were high estimates of additive genetic effects and estimates of common environmental effects, whose 95% confidence intervals did not include 0. Estimates of common environmental effects were larger with decreasing prevalence.

The summary estimate of schizophrenia’s heritability was high (point estimate, 81%; 95% confidence interval, 73%-90%). However, there were small but significant common environmental effects on liability to schizophrenia (point estimate, 11%; 95% confidence interval, 3%-19%).

This evidence shows that schizophrenia is inheritable.

Limitations include the fact that the quality of the twin studies was not uniformly high. Another limitation is the twelve studies included in the study were strictly heterogeneous. This means there was not the variance of having homogeneous data as well.

Evidence 3

In a study conducted by Rikke Hilker, et al., two nationwide registers, the Danish Twin Register and the Danish Psychiatric Research Register, a sample of twins was identified, born within the period of 1951 to 2000 (N=31,524 twin pairs). They were followed until June 1, 2011 (Hilker et al., 2018). Proband-wise concordance rates and heritability were estimated via use of Liability threshold models – categorical outcomes where a large number of variables are summoned to find an overall “liability” score, where the outcome is determined by whether the latent score is larger or smaller than the threshold (Neale, 2005) – with inverse probability weighting – a statistical technique where statistics standardised to a pseudo-population different from that of the population of the data collected is calculated (Mansournia & Altman, 2016).

Table 1: Proband-wise concordance rate compared between monozygotic and dizygotic twins (Hilker et al., 2018).

Concordance Rate
Monozygotic Twins 33%
Dizygotic Twins 7%

Table 2: Heritability of schizophrenia (Hilker et al., 2018).
Heritability of Schizophrenia
Estimate 79%

Concordance rate is the percentage of pairs of twins or other blood relatives who exhibit a particular trait or disorder (“APA Dictionary of Psychology”, 2021).

Monozygotic twins are also known as identical twins (“Monozygotic Twins – an overview | ScienceDirect Topics”, 2021), whereas dizygotic twins are known as nonidentical twins (“Dizygotic Twins – an overview | ScienceDirect Topics”, 2021).

Table 1 shows it is much more likely for both twins to inherit the same disorder in the case their parents also have schizophrenia. As for monozygotic (33%) compared to dizygotic twins (7%), since monozygotic twins essentially have the same genetic code, it means that quite a few pairs of twins did not both inherit schizophrenia. Table 2 shows a high rate of schizophrenia heritability, showing genetics does play a role in development of schizophrenia.
Though it can be seen that most sets of twins tested had inherited the disorder from their parents, since the concordance rate of monozygotic twins is so low, and the concordance rate of dizygotic twins is even lower, it shows heritability of schizophrenia is not entirely reliant on genetic factors.
Limitations to this are that the raw data is not provided. Since it could not be analysed, the calculations may be incorrect, and trends could not be easily identified. Similarly, it is unknown how heritability was calculated; therefore, it could be incorrect, and the data could not be satisfactorily analysed. Another limitation of the data is that only Danish monozygotic and Dizygotic twins were followed, meaning the data is possibly only relevant to Denmark.

Quality of evidence

Evidence 1 was not found on a website, but through google scholar searches, a link for the book was presented. While it was not a full text version, it provided enough information that the data could be interpreted and analysed. The book was in German, so there may be some issues in translation for some baseline information, but otherwise the document was comprehendible. There was minimal information about the author, however, searching for the author’s name proves him or her to be credible, given that they are a psychiatrist. The date was provided, however, the evidence was not current.

Evidence 2 was on a website that provided specifically for scientific journals and articles. There was no information about the author, and the date was recent. The purpose of the site is unbiased; however, to download some documents, you must create an account or pay. Sources were all referenced correctly. Similarly, many different studies were included in the data and report, meaning there was a large range of evidence from varying places, times, and demographics.

Evidence 3 was provided on a website that clearly identified its purpose. However, it only showed small parts of the study that could determine some trends, but not enough for sufficient analysis. There was no information on the author, but the study was recent. All sources were referenced correctly. Evidence 3 also had a large sample size, meaning the data was diverse, making the data more reliable.

Each piece of evidence tests completely different times. While evidence 2 and 3 both show studies on twins, one was from 2003 while the other was from 2018. Evidence 1 was from 1923, almost 100 years in the past.

Conclusion

Evidence 1 showed that schizophrenia in the tested families appeared in a large percentage of the children. This implies that genetics play a major role in the heritability of schizophrenia. Evidence 2 showed that schizophrenia is inheritable, and development factors also include external environmental influences. Evidence 3 showed that while heritability of schizophrenia is estimated to be quite high, the concordance between monozygotic twins is quite low, and implies that genetics is not the only factor in the heritability of schizophrenia.

From the evidence analysed in this investigation, it can be concluded that while psychological disorders are not entirely hereditary, genetics play a role in increasing the risk a child born to apparent with a psychological disorder has to develop said disorder, in comparison to the general population.

Extrapolation of evidence

Extrapolating evidence for the claim psychological disorders are inherited, it is suggested that schizophrenia is inherited. Although there is some evidence suggesting there are other factors contributing to development of schizophrenia, none of the evidence suggests that schizophrenia is not inherited.
Extensions and Improvements

An extension of the investigation is to further research heritability of schizophrenia in twins, and in relation to the parents. Another could be to research more specific aspects of heritability, where considering possible environmental factors that may come into play, such as stress or living situation, and increased chances of developing schizophrenia with schizophrenic parents.

An improvement is to use data where every participant was given a definite diagnosis of a psychological disorder. Another is to use a more understandable pedigree to display the data. More improvements include testing more families and individuals, especially from different countries, as it would provide more varied data; using data more uniformly high in quality; and to use more data where the studies are homogeneous, rather than strictly heterogeneous.

Further improvements include using evidence providing raw data to use for analysis, and data showing specifically how heritability was calculated.

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Tutor feedback:

This research investigation into the heritability of schizophrenia is quite thorough and commendable for its effort to integrate diverse pieces of evidence. However, there are several points worthy of further discussion and critique, which could enrich the analysis and interpretation of the findings.

Firstly, the use of DSM-V as a guideline is appropriate and sets a solid foundation for defining and diagnosing schizophrenia. It is critical, however, to acknowledge that diagnostic criteria and conceptualisations of psychological disorders, including schizophrenia, evolve over time. The reliance on DSM-V provides a contemporary framework, but it’s beneficial to recognise the historical context of diagnosis and its implications on understanding heritability.

The examination of the definition and factors considered in diagnosing psychological disorders by the DSM-V is valuable. It illustrates the complexity of defining and understanding these disorders, underscoring the multidimensional factors that contribute to their manifestation, including genetic, environmental, and psychobiological dysfunction. Implementing such a nuanced approach in analysing the heritability of schizophrenia enriches your investigation.

Your rationale highlighting the diverse definitions of psychological disorders, incorporating factors like distress, disadvantage, inflexibility, and statistical deviation, effectively sets the stage for examining schizophrenia. This background is crucial for readers to comprehend the complexities involved in attributing heritability to such disorders.

The exploration of heritability definitions is notably insightful, providing a foundation to understand how genetic variance can influence phenotypic variance in the context of schizophrenia. This dual perspective on heritability (statistical and common definitions) is essential for a nuanced analysis of the genetic underpinnings of schizophrenia.

The evidence provided from various studies, including the pedigree analysis of families and meta-analysis of twin studies, offers a compelling argument for the genetic components contributing to schizophrenia. Nonetheless, acknowledging the limitations of these studies is critical. The pedigree method, for instance, has its constraints in accurately representing genetic transmission due to the complexity of the genetic and environmental interactions in psychological disorders. Similarly, the sample size and selection, reliance on historical data, and potential biases in interpreting familial transmission of schizophrenia can significantly influence the conclusions drawn.

Your discussion on monozygotic and dizygotic twins in the context of schizophrenia provides a potent argument for the genetic component of the disorder. However, the relatively lower concordance rates among monozygotic twins than might be expected for a condition with a strong genetic basis pose important questions about the role of environmental and non-genetic factors.

The commentary on the limitations of the studies cited, including potential biases, methodological constraints, and the homogeneity of the data, reflects a critical and thoughtful approach to interpreting research findings. However, a more in-depth analysis of how these limitations might impact the overall conclusions about the heritability of schizophrenia would provide a richer, more nuanced understanding of this complex issue.

In conclusion, this investigation presents a well-considered exploration into the heritability of schizophrenia, making a significant contribution to the academic discourse on the genetic underpinnings of psychological disorders. Further enquiry into the interplay between genetic predispositions and environmental influences, along with methodological improvements in future studies, could offer more definitive insights into the heritability of not only schizophrenia but also other psychological disorders. Moreover, incorporating a broader discussion on the implications of these findings for clinical practice, public health policy, and the social understanding of schizophrenia would substantially enhance the practical and scholarly impact of your research.

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