Psychiatric disorders are commonly found to be influenced by a multitude of genes (SNPs, CNVs, etc.), often dozens or even hundreds.  Each genetic factor accounts for only a small part of the difference between affected and normal groups.  Frequently, perhaps mostly, the proportion is &lt;5%, often as low as &lt;=1%.  As the article says, there are many genetic and environmental factors involved in complex conditions like depression or schizophrenia, so these low levels of effect are unsurprising.<p>Here&#x27;s one article (and there are a great many more) describing such a finding:
<a href="http:&#x2F;&#x2F;www.ncbi.nlm.nih.gov&#x2F;pmc&#x2F;articles&#x2F;PMC3923972&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.ncbi.nlm.nih.gov&#x2F;pmc&#x2F;articles&#x2F;PMC3923972&#x2F;</a>