Thursday February 4 2010
It is important to remember that bipolar disorder is rare
“You don't have to be bipolar to be a genius – but it helps,” according to The Independent. The newspaper said that a Swedish study of over 700,000 adults found that those who scored top grades at school were “four times more likely to develop bipolar disorder than those with average grades”.
This study had strengths, including its large size, good sample selection methods and use of standardised data from national school exams. However, there were some limitations, including the fact that the researchers could not adjust for the influence of some factors that could have affected results, such as family history of bipolar disorder (previously known as manic depression). This means it is possible that some other factors might be behind the link seen.
Although this study suggested that those who achieved the highest grades may be at an increased risk of bipolar disorder later in life, it is important to remember that bipolar disorder is rare, even among high achievers.
Where did the story come from?
Dr James H MacCabe and colleagues from King’s College London and the Karolinska Institute in Sweden carried out this research. The study was funded by the Swedish Council for Working Life and Social Research, and the lead author was supported by the UK Department of Health and the Medical Research Council. The study was published in the peer-reviewed British Journal of Psychiatry.
The Independent and The Daily Telegraph have both reported on this research. Although their coverage is generally accurate, they have reported the risk in terms of relative increases, saying that “clever children are almost four times more likely to suffer from manic depression”. While a four-fold increase in risk may sound large, this does not reflect that the chance of developing bipolar disorder, even for those with higher school achievement, is itself quite low.
What kind of research was this?
This was a cohort study looking at whether there was a link between academic performance in school and the risk of developing bipolar disorder later in life. It looked at academic performance in national exams at age 16 and data on the individuals’ mental health for the next decade. The researchers say that although belief in a link between 'genius' and mental health problems has existed for a long time, few research studies have looked at the possibility of a link.
Cohort studies are good for looking at the link between factors that cannot be studied through randomised controlled trials. This study used data on all individuals who finished compulsory education in Sweden over almost a decade. The size of the dataset available and the fact that it was likely to have included the majority of individuals aged 16 in the country means that the sample is less likely to be biased, and should be a good representation of the Swedish population as a whole.
The data analysed in this study were collected prospectively. This means the figures were recorded as the events occurred, which is preferable to asking people to remember what happened in the past. This practice increases the likelihood that the study’s data are accurate. However, with all studies of this type it is important that researchers take into account factors that could affect the results (potential confounders). In this case, the data used were not originally collected specifically for this study, and therefore may not have recorded some types of information that the researchers might have liked to collect about potential confounders. Having data collected by many different healthcare professionals also means it may not have been collected in the same way for all individuals.
What did the research involve?
The researchers obtained school results for all individuals who finished compulsory education in Sweden between 1988 and 1997. The researchers then looked at the medical records of these people to identify anyone who had subsequently been admitted to hospital for bipolar disorder.
The researchers obtained data for their study from national registries. Information on school performance came from the Swedish national school register, which records this information for all pupils graduating from compulsory education at the age of 16. The researchers say that most pupils with intellectual disabilities or sensory impairments are integrated into mainstream education in Sweden and are therefore included in the register.
The researchers obtained the students’ grades in 16 compulsory subjects, which were based on performance in national examinations sat when they were 16 years old. These exams are graded in a standard way, and the results are combined to give each student a grade point average. Information on admissions to hospital for psychiatric disorders was obtained from the Swedish hospital discharge register, which contains details of hospital stays and diagnoses. Other registers were used to collect information on the individuals’ parents, such as their socio-economic status, education, citizenship and country of origin.
In their analyses the researchers excluded people who had a parent born outside of Sweden as they were more likely to have missing data, and migrant status might have affected the results. They also excluded people who were hospitalised for any psychotic disorder before their exams or in the year after their exams. This left 713,876 individuals, who were followed to December 31 2003. On average, participants were aged 26.5 years at the end of the follow-up period.
The researchers standardised individuals’ school performance using an accepted method that looks at how far away their grade point average is from the average score for their gender. They then analysed the relationship between the overall level of performance in the exams and risk of bipolar disorder. They also looked at the relationship between performance in individual subjects and bipolar disorder, comparing those who got an ‘A’ grade in each subject with those who got ‘B to D’ grades.
The researchers took into account factors that could affect the results (potential confounders), such as gender, season of birth, paternal or maternal age over 40 years at individual’s birth, parental socio-economic status and parental education.
What were the basic results?
During the follow-up period 280 people developed bipolar disorder. This equates to around four people out of every 10,000 people developing bipolar disorder over 10 years.
The researchers found that those people who had excellent grades were just over three times more likely to develop bipolar disorder than people who had average grades in school at age 16 (hazard ratio [HR] after adjustment for potential confounders 3.34, 95% confidence interval [CI] 1.82 to 6.11).
When the researchers looked at men and women separately, the link between better school performance and bipolar disorder was stronger in men, but the difference between the sexes was not statistically significant. People who had the poorest grades in school were also at an increased risk of developing bipolar disorder compared to people with average grades (adjusted HR 1.96, 95% CI 1.07 to 3.56).
When looking at performance in individual subjects, scoring A grades in childcare, Swedish, geography, music, religion, biology, history and civics was linked to an increased risk of bipolar disorder. The link with other subjects was not as strong. Those scoring an A grade in sport were less likely to develop bipolar disorder than those who got B to D grades.
How did the researchers interpret the results?
The researchers conclude that their results “provide support for the hypothesis that exceptional intellectual ability is associated with bipolar disorder”.
This large study suggested that those achieving the highest or lowest grades at school at age 16 were at a higher risk of developing bipolar disorder than students with average performance. There are a number of points to consider when interpreting this research:
- Although the fact that data were collected prospectively increases the reliability of them, some data may be missing, recorded incorrectly or inaccurate.
- Data on diagnoses were based on information recorded at hospital discharge. As the same doctors did not assess all patients, there could have been variation in how bipolar disorder was diagnosed. In addition, any people who had bipolar disorder but had not been hospitalised would not have been identified.
- As with all studies of this type, results may have been influenced by factors other than those assessed. Although the researchers took some of these factors into account, other unmeasured or unknown factors could be having an effect. For example, the researchers did not have information on whether there was any family history of bipolar disorder, or about life circumstances in adult life, and therefore could not take their influence into account.
- The study only followed people to an average age of about 26, a longer follow-up period might show different results.
- It is possible that the link between school performance and bipolar disorder arises because people with higher school achievement or their families are more likely to seek treatment if they experience symptoms of bipolar disorder. However, the authors suggest that this does not seem to be the case, as their previous research found that higher school achievement was associated with reduced risk of schizophrenia and schizoaffective disorder.
- The analysis phase looking at individual subjects was not the main focus of the study and involved multiple statistical tests. This can increase the likelihood of findings occurring by chance, and on this basis, these results should be seen as tentative.
The results of this study do not mean that very high or low school performance actually ‘causes’ bipolar disorder, only that there was an association between the factors in the population studied. One potential explanation suggested by the researchers is that certain aspects of how the brain works in bipolar disorder could also be related to creativity or school performance.
It is important to realise that bipolar disorder is rare, with this study finding that only four cases developed per 10,000 people during a follow-up period of 10 years.