© AlamyWhen people go through a traumatic experience it affects their cells and that change can be passed on to their children.
Psychologists have known for some time that trauma can cause behavioural disorders, such as depression, which can be passed down from one generation to the next.
"There are diseases such as bipolar disorder, that run in families but can't be traced back to a particular gene", said Prof Isabelle Mansuy at the University of Zurich.
Now researchers have found that exposure to high levels of stress alters the production of 'microRNA' molecules, which help regulate genes.
And they were found to be present in sperm, suggesting that they could be passed on to future generations.
Mice exposed to high levels of stress were seen to exhibit depressive symptoms and their metabolism slowed down.
Those behavioural symptoms were also seen in their offspring even though the mice were not exposed to any traumatic stress themselves. The changes were ever found in third generation mice.
"We were able to demonstrate for the first time that traumatic experiences affect metabolism in the long-term and that these changes are hereditary," said Prof Mansuy.
"With the imbalance in 'microRNAs' in sperm, we have discovered a key factor through which trauma can be passed on.
"Most likely, it is part of a chain of events that begins with the body producing too much stress hormones."
After traumatic experiences, the mice behaved markedly differently: they partly lost their natural aversion to open spaces and bright light and had depressive-like behaviours.
Mansuy and her team are now studying the role of 'microRNAs' in humans to see if the results would be replicated.
If it was possible to demonstrate that an imbalance in the blood was causing the trauma a test could be developed to diagnose inherited depression.
The study was published in the journal Nature Neuroscience.
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