What have twin studies revealed about the causes of schizophrenia?
Schizophrenia has been diagnosed in around 1% of the population (Workman, 2012), which means that it affects a great number of people globally. Therefore, it is important to find more out about the causes of Schizophrenia to aid developments in areas of diagnosis and in treatment.
One of the most frequently used methods for studying the causes of Schizophrenia is of twin studies, where monozygotic (identical) twins are compared with dizygotic (non-identical) twins to look for concordance rates of a particular characteristic or disorder to determine whether there is a biological or environmental link for those characteristics or disorders
This essay will investigate what these studies have revealed about the possible causes of Schizophrenia and whether these findings fall on the nature or the nurture side of the debate. Some will argue that the disorder has a clear genetic link and therefore is pre-determined, while others would say that our environment is what shapes us and may cause Schizophrenia. A final argument would say that is a combination of both environmental and biological factors that cause the disorder; a multifactorial system.
A definition of Schizophrenia
Although there are some disparities in the definition of Schizophrenia globally, there are some elements of the disorder which are frequently noted. Schizophrenia is a disorder of thinking where a persons ability to recognise reality, his or her emotional responses, thinking processes, judgement and ability to communicate deteriorates so much that his or her functioning is seriously impaired. (Birchwood & Jackson, 2001) Therefore Schizophrenia is considered a psychotic illness under the psychosis heading, rather than the neurosis heading, based on the fact that for a disorder to be a form of psychosis, a person can no longer distinguish what is reality and the symptoms that they are experiencing.
Some of the major symptoms of Schizophrenia can be divided between positive and negative symptoms. Positive symptoms include behaviours which are in addition to normal function, with common examples being auditory or visual hallucinations, disorganised speech, delusions , otherwise known as false beliefs whereby the sufferer believes they are someone they are clearly not (such as a member of the royal family (Workman, 2012), as well as also movement disorders, for example, repetitive movements or catatonia, which is a state in which a person does not move and does not respond to others. (National Institute of Mental Health, 2009)In comparison negative symptoms are when a behaviour associated with normal functioning is no longer present. Symptoms include: social withdrawal, little or no speech, flat affect or general apathy.
Schizophrenia is a complex disorder which can be divided into sub categories. This is relevant to the study of twins as a concordant set of twins may not be diagnosed with the same type of Schizophrenia. Some of these sub types include paranoid schizophrenia, catatonic schizophrenia and disorganised schizophrenia.
Furthermore, Schizophrenia usually develops in stages; the first of these stages is the Prodromal phase, when a person will begin to show signs of Schizophrenia, which typically will include isolation, lack of motivation and willingness to participate in activities. However, it is clear that these behaviours may be due to other emotional factors, general teenage behaviour or may relate to other disorders such as depression, Until a patient experiences psychotic symptoms, a physician cannot diagnose schizophrenia. (Veague, 2009) This is why Schizophrenia is difficult to diagnose and highlights how twins studies could useful; knowing how one twin experiences the prodromal stage may make it easier to spot the onset of the disorder in the other twin. Furthermore, comparisons on the effect of the environment on how they experience the start of the disorder can be investigated. The stages after the prodromal period include the acute stage and the residual stage.
Schizophrenia is not necessarily a long term disorder, yet many people who have suffered with the disorder suggest the stigma and distress of Schizophrenia will affect them throughout their lives. It is believed that the chance that Schizophrenia will be a lifelong condition is 1% and between 20% and 25% will have one episode with full remission. (Birchwood & Jackson, 2001)This means that these peoples experience of the illness can most valuable as it can show the effectiveness of the treatment they were given. Schizophrenia is typically treated using antipsychotic medication and / or psychosocial treatments such as Cognitive Behavioural Therapy (CBT) or rehabilitation.
DSM IV definition of schizophrenia
A. Characteristic symptoms: Two (or more) of the following each present for a significant portion of time during a 1-month period (or less if successfully treated):
(3) disorganised speech (e.g., frequent derailment or incoherence)
(4) grossly disorganised or catatonic behaviour
(5) negative symptoms, i.e., affective flattening, alogia, or avolition.
Note: only one Criterion A symptom is required if delusions are bizarre of hallucinations consist of a voice keeping up a running commentary on the persons behaviour or thoughts, or two or more voices conversing with each other.
B. Social/occupational dysfunctions: For a significant portion of the time since the onset of the disturbance, one or more major areas of functioning such as work, interpersonal relationships, or self-care are markedly below the level of achieved prior to the onset (or when the onset is in childhood or adolescence, failure to achieve expected interpersonal, academic, or occupational achievement).
C. Duration: Continuous signs of the disturbance persist for at least 6 months. This 6-month period must include at least 1 month of symptoms (or less if successfully treated) that meet Criterion A (i.e., active phase symptoms) and may include periods of prodromal or residual periods, the signs of the disturbance may be manifested by only negative symptoms or two or more symptoms listed in Criterion A present I an attenuated form (e.g., odd beliefs, unusual perceptual experiences).
D. Schizoaffective and mood disorder exclusion: Schizoaffective disorder and mood disorder with psychotic features have been ruled out with (1) no major depressive, manic, or mixed episode have occurred concurrently with the active-phase symptoms; or (2) if mood episode have occurred during active-phase symptoms, their total duration has been brief relative to the duration of the active and residual periods.
E. Substance/general medical condition exclusion: The disturbance is not due to the direct physiological effects of a substance (e.g., a drug of abuse, a medication) or a general medical condition.
F. Relationship to a pervasive development disorder: If there is a history of autistic disorder or another pervasive development disorder, the additional diagnosis of schizophrenia is made only if prominent delusions or hallucinations are also present for at least a month (or less if successfully treated).
ICD diagnostic criteria for schizophrenia
A. One of the symptoms under A1
Or two of the symptoms under A2
Must be present for most of the episode lasting at least a month
A1a. Thought echo, insertion, withdrawal, broadcasting
A1b. Delusions of control, influence, passivity, delusional perception
A1c. Verbal hallucination, with running commentary or discussing patients or coming from a part of the body
A1d. Delusions that are persistent and culturally implausible
A2e. Persistent hallucinations with half-formed delusions with clear affective content.
A2f. Breaks in train of thought, giving rise to incoherent or irrelevant speech, neologism
A2g. Catatonic behaviour
A2h. Negative apathy, paucity of speech, blunted or incongruent affect, not due to depression or medication
B. If manic or depressive episode, Criterion A must be met before mood disturbance developed.
C. Not attributable to organic brain disease (FO) or substance abuse (F1).
All three conditions, A, B and C must be satisfied.
Figure 1: the DSM IV and the ICD 10 definitions of schizophrenia, which highlights the complex components of the disorders as well as the number of factors that can be considered in diagnosis (Birchwood & Jackson, 2001, pp. 11-12)
Twin studies themselves have been used for many years. In the early 19th Century when the psychiatrist Benjamin Rush commented on the usefulness of twins in study mental illness. Since then many researchers have used twins as participants for studies on development, mental illness, language, cognition and learning difficulties. One of the most infamous researchers of twins was Josef Mengele, who worked in Auschwitz during the holocaust and who was particularly interested in discovering information on how twins are different to the rest of the population. His procedures on twins ranged from attention and memory where a twin would be required to do reading aloud and remembering the sentence read as well as looking at twins spiritual development. (Strzelecka, 2011) However, his work could often be distressing and cause much pain to his victims, with one set of twin so much blood was drawn that they died. (Strzelecka, 2011) During his time at Auschwitz Mengele, identified approximately 1,500 twin pairs, (Fuller Torrey, et al., 1994) This highlights the scale of his work, which demonstrated to others the benefits of using twins in research. Furthermore, st