It’s like something from string theory or deja vu all over again. Science has long known that psychological disorders such as bipolar and depression are passed to the next generation, as if through reincarnation. But until now, not much has been known about how psychological stress changes the body in this life and the next.

These multi-generational phenomena — i.e., “his dad was nuts, too” — cannot be explained by mere genetics, according to Isabelle Mansuy, a researcher at the University of Zurich. For years, she has studied the molecular underpinnings of these non-genetic transferences of behaviors caused by traumas endured early in life.

Biologically speaking, it’s not that the son suffers the sins of the father. They both suffer.

"There are diseases such as bipolar disorder, that run in families but can't be traced back to a particular gene,” Mansuy said in a university news release.

The researchers achieved success in identifying short RNA molecules as an integral part of such heritability. In the process, enzymes fashion these short RNA molecules by following a portion of the genetic directions — encoded on DNA — as a template. These resulting RNAs are then “trimmed” into mature form by other enzymes, ready to serve a regulatory function in the molecular human world.

In the larger world of laboratory mice, researchers found that adults exposed earlier in life to trauma experienced differing levels of short RNA production, with some running high and others running low compared to others who’d led more carefree lives. In the lab, mice exposed to trauma earlier in life behaved noticeably different in comparison to others, losing their natural aversion to open spaces — somewhat “suicidal” in the mouse world — and displaying behaviors consistent with depression.

These symptoms were then passed to the next generation not by genetics per se but through the father’s sperm. With absolutely no personal experience with trauma, an individual may feel the effects of an intergenerational hurt, it seems.

Aside from depressive behaviors, the mice with trauma history suffered an impaired metabolism with lower levels of blood-sugar and insulin than others. "We were able to demonstrate for the first time that traumatic experiences affect metabolism in the long-term and that these changes are hereditary,” Mansuy said.

From shirtsleeves to shirtsleeves in three generations, as the old saying goes. Similar to other studies in the emerging field of epigenetics, the researchers found effects on not only behavior but metabolism.

"With the imbalance in ‘microRNAs’ in sperm, we have discovered a key factor through which trauma can be passed on,” Mansuy said.

Yet many aspects of the process remain mysterious to researchers, including how the dysregulation in short RNAs actually happens — which Mansuy speculatively describes as a “chain of events” born of an overproduction of stress hormones.

Although we cannot prevent the past, Mansuy and other epigeneticists hope to diagnose what ails us now and into the future. Among interesting avenues of research is the tantalizing idea that behavioral traits may be inherited through similar non-genetic mechanisms building on a skeletal frame of DNA.

Source: Gapp K, Mansuy I, Prados J, et al. Implication of sperm RNAs in transgenerational inheritance of the effects of early trauma in mice. Nature Neuroscience.  2014.