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 <5%, often as low as <=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.

Here's one article (and there are a great many more) describing such a finding: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3923972/