Have you ever wrongly suspected that other people are out to harm you? Have you been convinced that you’re far more talented and special than you really are? Do you sometimes hear things that aren’t actually there?



These experiences – paranoia, grandiosity and hallucinations in the technical jargon – are more common among the general population than is usually assumed. But are people who are susceptible simply “made that way”? Are they genetically predisposed, in other words, or have their life experiences made them more vulnerable to these things?



It’s an old debate: which is more important, nature or nurture? Scientists nowadays tend to agree that human psychology is a product of a complex interaction between genes and experience – which is all very well, but where does the balance lie? Scientists (including one of the authors of this blog) recently conducted the first ever study among the general population of the relative contributions of genes and environment to the experience of paranoia, grandiosity and hallucinations.

How did we go about the research? First, it is important to be clear about the kinds of experience we measured. By paranoia, we mean the unfounded or excessive fear that other people are out to harm us. Grandiosity denotes an unrealistic conviction of one’s abilities and talents. Hallucinations are sensory experiences (hearing voices, for instance) that aren’t caused by external events.



Led by Dr Angelica Ronald at Birkbeck, University of London, the team analysed data on almost 5,000 pairs of 16-year-old twins. This is the classical twin design, a standard method for gauging the relative influence of genes and environment. Looking simply at family traits isn’t sufficient: although family members share many genes, they also tend to share many of the same experiences. This is why studies involving twins are so useful.

Fraternal (“dizygotic”) twins develop from separate eggs that have been fertilised by separate sperm. Like all siblings, fraternal twins share about 50% of their genes. Identical (“monozygotic”) twins, on the other hand, are the result of the fertilisation by a single sperm of one egg that subsequently splits into two. This means that their genetic makeup is exactly the same. So if rates of a particular psychological trait are more alike in identical twins than fraternal twins, we can be pretty certain that this is gene-related.



What did the analysis reveal? Heritability for paranoia was 50%; for grandiosity it was 44%; while for hallucinations it was 15% for males and 32% for females. This doesn’t mean, incidentally, that 50% of an individual’s paranoia is the result of their genes. The concept of “heritability” tells us that 50% of the differences in levels of paranoia across the population may be genetic in origin. Heritability statistics tell us nothing about individual cases – they describe broad trends across a large group. The remainder of the differences between people are the product of environmental factors – essentially everything apart from our DNA, including the experiences we have gone through during our lifetime, but also potentially some biological factors too.



What the study shows then is that, although genes play a part in these kinds of experiences, the environment is generally more important. Whether we have them depends as much, if not more, on what happens to us during our lives as it does on the genes with which we were born.



The study has intriguing implications for the understanding of schizophrenia. At their most severe and persistent, paranoia, grandiosity and hallucinations are usually taken as symptoms of the illness – a disorder that has typically been regarded as highly heritable. But the research casts doubt on the view that these experiences should be lumped together under this diagnosis. The figures showed only a moderate association between the incidence of paranoia and hallucinations (genes accounted for much of the overlap). Grandiosity was even less likely to crop up alongside either of the other two experiences.

This isn’t what one would expect from symptoms of the same underlying illness. Indeed, the results as a whole lend weight to the increasingly influential argument that “schizophrenia” is actually a vague, catch-all term for a variety of relatively independent psychological problems.



Twin studies of those diagnosed with schizophrenia typically report much higher rates of heritability for the disorder, but there’s an inherent bias in data drawn only from those seen in hospital or via psychiatric services. By definition, these are people with many and complicated problems. Because each of those problems independently increases the chance of hospital admission, the more you have the more likely you’ll end up in services. It’s easy to assume that the symptoms must therefore be interrelated. In fact, the only common factor in these patient studies may be the presence of severe social difficulties. Perhaps this social dysfunction is what the higher heritability estimates have been capturing, and not the psychotic experiences themselves.



Schizophrenia is rarely diagnosed in people as young as those in our study. Nevertheless, what we see here is a new way of approaching the illness. Instead of getting bogged down in problematic diagnostic categories, the focus is switched to the psychotic experiences themselves. Paranoia, grandiosity and hallucinations aren’t confined to those with serious mental health problems. Like feelings of sadness or anxiety, they exist on a spectrum of severity in the population. And these experiences, the research shows, are far more likely to be triggered by environmental factors than we might previously have supposed.



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