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This can be difficult to ascertain due to the lack of insight that patients have, the differing measures used, small sample sizes, etc.. But despite this, some information can be gleaned.

 

When we consider the elderly, older community studies reported a prevalence of 1.7% for late onset schizophrenia to 4% for persecutory delusions. A recent review of 5,222 patients, as part of the MRC ALPHA study, looked at 3,519 patients two years later and those with schizophrenia and delusional disorder were then reinvestigated two years later on. At baseline the prevalence of delusional disorder was 0.04 (95% CI 0.04-0.14); the incidence at year 2 was 15.6 per 100000 (95 CI 0.02-135.1), though the absolute numbers were small (three at baseline and two in year 2 (Copeland 1998). Other prevalence estimates range from 10-77%, with other studies mentioning figures of about 30%. When we consider those with Mini Mental State Scores greater than or less than 24, the prevalence rate was about 4-6%. It has also been estimated that 5% of the normal elderly have delusions. The commonest types of delusions in dementia are persecution and misidentification, with the risk increasing with age and tending to start in the 70s. Risk factors include impaired hearing, antihypertensive, heart attacks, congestive cardiac failure and being female (Holt 2006).

 

Interestingly there is evidence that the prevalence of delusions may be greater in the non-patient population. Figures range from 1-3%, with the figure rising to 5-6% when the severity criteria are lessened. About 10-15% have delusional ideation, with figures depending on study design. 10-15% may hold paranoid ideation, though as this people often deny them the figure could be higher (Freeman 2006). Self-reported figures may differ from clinically rated figures in the non-clinical population, e.g. in the Dunedin cohort 25% reported at least 1 delusional or psychotic symptom but only 3.7% fulfilled clinical criteria. The National Survey of Psychiatric Morbidity in Great Britain found that 9.1% of people responded positively to “have there been times when you felt that people were deliberately acting to harm you or your interests?” and 1.5% endorsed “have there been times when you felt that a group of people were plotting to cause you serious harm or injury”, with a multivariable model associating neuroses, past victimisation, young, alcohol dependence, recent life event, average IQ and being male with paranoid thoughts (Johns 2004).

 

When looking at non-demented elderly people aged 85 in Sweden, 2% had delusions as found in the psychiatric examination, but 4.6% were delusional if we considered the key informant interview but only 0.6% had delusions mentioned in their notes. 5.3% of women and 5.8% of men were delusional. In the whole sample delusions were related to depression, blunted affect and paranoid personality traits (Ostling 2002).

 

Another study looked at 444 controls and 33 deluded patients and gave them the PDI-21. The main outcome measure was the sum of yes / no answers to each PDI item. The deluded group had higher total / distress / preoccupation and conviction scores, but there was significant overlap in scores between the controls and deluded group. On looking at the total sample, there was an inverse correlation between PDI score and age (Peters 2004). When we come to children, there is also evidence of delusional ideation. A birth cohort from the Avon Longitudinal Study of Parents and Children looked at 6455 children and assessed the presence of delusions using the PLISKi instrument. Self-report data showed prevalence’s of 1.2% for thought withdrawal to 16.3% for being spied on and these scores were higher than observer rated data (Horwood 2008).

 

What is the difference between those with ‘delusional beliefs’ in the clinical and non-clinical populations? The answer in part is distress. Those with religious convictions tend to believe firmly with less distress than a comparable clinical population. This distress felt by patients may be enhanced by negative self-appraisals (Kuipers 2006).

 

The 2000 British National survey of psychiatric morbidity screened 8580 patients between the ages of 16 – 70 and identified psychotic symptoms. The psychosis screening questionnaire was used, with a probe question and a more specific question afterwards e.g. thought insertion, paranoia, strange experiences, etc. For these three the prevalence of a yes for the probe question was 9%, 21.2% and 8.9% respectively, with the figures for the specific question being 0.9%, 1.5-9.1% and 3% respectively. The analysis concluded that paranoia was associated with neurosis, victimization, being young, drinking alcohol, having life events in the last 6 months and being male. This highlights the gender and age issues and also shows that the prevalence depends on the question asked (Johns 2004).

 

Information about delusional ideation in primary care patients has also been looked at. A study of French primary care patient’s gave patients the PDI-21 to fill. The main outcome measure was the number of positive answers. 90% of the patients were European. The total PDI score was 4.3 (SD = 3.3) with mean distress being 9.4 (SD=9.4), preoccupation 9.8 (SD=9.3) and conviction 13.8 (SD=11.4). When we consider those with psychoses v those with no mental illness, the PDI score was higher in the former, as was distress (8.38 v 19.31, p=0.0006), preoccupation (9.08 v 15.44, p=0.05) and conviction (12.91 v 25, p=0.008). This argues against the point in Johns 2004 that delusions may be commoner in the general population. The psychotic patients tended to endorse persecutory, mystic and guilt items. When they looked at psychotics v non-psychotics, the scores were higher for the former (7.6 v 4.9, p=0.05) with items being endorsed more tending to be persecutory, conspiracy, closeness or chosen by god and sinned more than the average person (Verdoux 1998).

 

A review of PDI scores in 2441 Australians aged between 18 – 23, who were born to women in the Mater university of Queensland study of pregnancy, found a mean age of 19.9 year (SD=0.87), 10.5% had endorsed delusional measures on the CIDI, with 5.9%e endorsing 1 item and 2.4% for 2 items and 2.2% endorsed more than 3 items. Those who endorsed delusional items were more likely to experience hallucinations (OR 4.78 95%CI 3.43 – 6.66) and those with auditory hallucinations were more likely to be delusional (OR 10.41 95%CI 6.73 – 16.13). Those who were older were less likely to endorse delusional items (OR 0.66 95%CI 0.48 – 0.92), which present after controlling for education (OROR 0.65 95%CI 0.47 – 0.90). Removing those with schizophrenia, brief psychosis, schizophreniform / delusional disorder, confirmed the age effect. There was no effect on gender on CIDI delusional scores. Scores on the PDI rated from 5.5% to 77%, with similar gender prevalence after control for education, but not all: - women endorsed items 9, 12 more and males endorsed 6 and 10. Younger patient’s rated higher on PDI scores and females were more likely to have highest scores. Those who endorsed CIDI hallucinations / delusions were more likely to have higher PDI scores. 7.5% had delusional experiences in the last 12 months, especially if they had visual or auditory hallucinations (Scott 2008).

 

An interesting study looked at the prevalence of delusions in a non-religious group, Christian group, a new religious movement (NRM) and a deluded group. The average PDI score, distress / preoccupation / conviction ratings were higher deluded group as compared to the 1st 2 groups but were comparable to the NRM group. Differences tended to be that the NRM group had less distress. The authors argue that analysing delusional dimensions may be more important that content. But there was a large overlap in scores between the 4 groups (Peters 1999).

 

The application of the 40 item PDI to a group of 272 students and 35 psychotic patients, did not find a difference in distress, conviction, preoccupation and total scores between the genders. There was a significant difference between the deluded and healthy patients in terms of the total, distress, conviction and preoccupation scores, but there was no difference in the median scores and the range of scores. Interestingly there were those in the healthy group whose mean scores were higher than the psychotic and deluded group. Interestingly the paper did not mention whether a pattern of responses of the PDI could separate the groups, e.g. 27 items showed higher distress scores, 30 items had high scores on preoccupation and conviction, a similar proportion of the deluded and health group felt that they were special or unusual(Peters 1999).

 

When we look at the prison population, there is abundant evidence that the psychoses rate is higher in prison. The British national survey of psychiatric morbidity looked at the rates of psychoses in prisoners and compared them to the general population. Part of the measures included looking at delusions. 3142 prisoners were interviewed, as were 10,108 members of the general population, of these 505 and 473 were assessed with the Schedule for Clinical Assessment in Neuropsychiatry. When it came to delusions, the percentages were 5% and 6% respectively. There was no difference between the groups in the other delusional measures SCAN section 18 / 19 (Brugha 2005).

 

When we look at the prevalence of delusional jealousy, a retrospective case examination of all psychiatric patients between 1981–1985 in Munich found that out of 8134 individuals, 93 had delusional jealousy. The prevalence rate in organic psychosis was 7%, in paranoid disorder 6.7%, alcohol psychosis 5.6% and schizophrenia 2.5%, with overall prevalence being 1.1%. There was gender equality in delusions of jealously but females predominated in schizophrenia and males in alcohol psychosis (Soyka 1991). The prevalence of normal jealousy is probably quite common and perhaps everyone has experienced this at some point in their lives (Crowe 1995).

 

In Parkinson’s disease, reports of prevalence has been estimated to be between 3 – 17%. Risk factors for psychosis in Parkinson’s include older age, larger doses or levodopa, anticholinergics, multiple drug therapy, dementia, cerebral atrophy and psychiatric conditions (Roane 1998).

 

Incidence of Delusional Misidentification Syndrome has often thought to be low. This has been challenged by some, but one particular review took all case reports of DMS up till 1994 and found 58 accounts of that included autobiographical details. Misidentification was defined as any “mistaken identification of persons”, with a broader definition (which weren’t defined) being used in addition to the normal DMS criteria. They found 3 cases (2 with Capgras and 1 with intermetamorphosis) and 16 who fitted a broad definition. Those without the typical DMS patterns tended to mistake strangers as familiar or famous people. Sometimes the explanations for these misidentifications were comforting (Mojtabai 1998).

 

Some studies have shown that rates of delusional disorder may be 40-50x times greater in immigrants and refugees, which may be due to the loss of social ties in the homeland and due to the difficulties in gaining new social ties in the new society. This may be present in individuals without pre-existing mental illness, which implicates social causation, though not all people had pre-existing mental illness. Delusional disorder is also associated low socioeconomic and educational level, with social causation being implicated by the higher levels of socioeconomic disadvantage being found than for schizophrenia (Hagen 1995).

 

The incidence of erotomania isn’t available for various reasons but it is a delusion which is found all around the world. As sexual behaviour has become more relaxed in the west, it is arguable that it would not meet the social incompatibility criteria for delusions.

 

Body dysmorphic disorder has a prevalence of 2% in the general population, in those wanting medical treatment the prevalence is about 5%, with rates being higher in those seeking cosmetic surgery. In depression, rates of 8% have been mentioned. The prevalence of Body Dysmorphic Disorder (BDD) may be about 1-2% but many are ashamed to admit there defects, so it probably is an underestimate. The delusional form of BDD is not known (Arthur 2007). Some community studies indicate 1.9% prevalence, to 3.2% in psychiatric outpatient’s (Carey 2004). Self-report and clinician reports differ and the distress felt may have cultural variations e.g. in Japan the concern is for the distress felt by observers. Even though there is equal gender reporting, men tend to focus on balding, their genitals and body shape and women focus on skin, hips, weight (Castle 2006).

 

Although prevalence measures are useful, many studies have used students, mainly for ease of access. But there is evidence that prevalence estimates in these populations are different from the general population. When 78 students were compared with a sample from a general population, students tended to have more delusions and to be more distressed and preoccupied by them, but not in an age matched group (Lincoln 2008).