What causes paranoia, hallucinations and grandiose ideas?

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 in our lifetime, but also potentially some biological factors too.


Of course there is research out there that says the opposite. This adds to the debate though.