A growing body of research has emerged in the past decade surrounding the heritability of trauma. It's an interesting topic that was once thought to be farfetched. How can the troubles of your grandparents be passed down to you when you haven't experienced them yourself?
The concept rose to prominence when a review by Natan Kellerman (2013) examined the effect the atrocities of the Holocaust had on three generations of survivors’ families. It was found that the children and grandchildren of survivors were particularly susceptible to stress when compared to the general population. Where theories had assumed such transmission was caused by environmental factors, such as the parents’ child-rearing behavior, Kellerman posited that a biological memory of an ancestor’s experience could be inherited.
However, unlike most inherited conditions, this was not caused by mutations to the genetic code itself. Instead, the traumatic experiences of the individual are thought to change how genetic changes are expressed in their children and grandchildren. This process is known as ‘epigenetics,’ where the expression of genes is modified without changing the DNA code itself.
Exploring this further, a 2015 study (Yehuda et al., 2016) assessed 32 Holocaust survivors and a total of 22 of their children, as well as a small control group consisting of 8 participants. They found that the children of the survivors of the Holocaust had epigenetic changes to a gene that was linked to their levels of cortisol, a hormone involved in the stress response.
Such a finding was extraordinary and has major implications; it would mean that experiences throughout your lifetime, particularly traumatic ones, would influence biological changes in your offspring for generations to come.
However, the study by Yehuda and colleagues was quickly criticized for a number of reasons. Firstly, the scientists studied blood, which is a mixture of cell types, making it difficult to ascertain the cause for the changes reported by participants. Secondly, the study only looked at a small subset of genes- a proverbial drop in the ocean of the human genome. Without studying several generations and searching more widely in the genome, we can’t conclusively say any changes were due to epigenetic inheritance.
While initially exciting, we must be cautious when interpreting evidence for epigenetics. The concept of our environment altering the expression of our genes is a well-established, evidence-based theory. Conversely, evidence for this process happening between generations is lacking and methodological flaws in research haven’t helped the cause.
This doesn’t necessarily mean we should rule out the epigenetic heritability of trauma entirely. Taking a slightly more empirical approach, controlled experiments have used mice to explore the intergenerational effect of trauma (Dias & Ressler, 2013). By taking this approach, researchers can control the environment and the breeding of the mice.
The study by Dias and Ressler showed an intergeneration effect of trauma associated with scent (Dias & Ressler, 2013). Seemingly channeling B.F Skinner and his infamous ‘Skinner Box’, researchers blew acetophenone (scent of cherry blossom) through the cages of adult male mice, zapping their feet with an electric current at the same time. Repeating this procedure several times, the mice associated the smell with pain.
Once the male mice had been conditioned to associate the scent with pain, they bred with female mice. Amazingly, when their offspring smelled the scent of acetophenone, they showed a more fearful response than offspring from mice who hadn’t been conditioned to fear the scent.
How we do know they hadn’t simply learned about the smell from their parents? They were raised by unrelated mice who had never smelled the scent. Yet another generation later, the ‘grandchildren’ of the original mice also showed heightened sensitivity to the scent.
Much like Kellerman’s observation that offspring were more sensitive to trauma, the second and third generation of mice did not appear to fear the scent itself, but rather had a heightened sensitivity to it. This is important as the offspring don’t always display the same trait as their ancestor developed.
Such findings are extremely exciting, albeit extremely rare, especially when studying mammals, including mice. The prospect of understanding how human attributes can emerge from our experience, or more tantalizingly, our ancestors, would be revolutionary and would have major implications for psychology, as well as numerous other medical fields. These mechanisms still aren’t well-understood but given time and patience, we may yet come to understand how our life experiences can shape the lives of our descendants.
The concept rose to prominence when a review by Natan Kellerman (2013) examined the effect the atrocities of the Holocaust had on three generations of survivors’ families. It was found that the children and grandchildren of survivors were particularly susceptible to stress when compared to the general population. Where theories had assumed such transmission was caused by environmental factors, such as the parents’ child-rearing behavior, Kellerman posited that a biological memory of an ancestor’s experience could be inherited.
However, unlike most inherited conditions, this was not caused by mutations to the genetic code itself. Instead, the traumatic experiences of the individual are thought to change how genetic changes are expressed in their children and grandchildren. This process is known as ‘epigenetics,’ where the expression of genes is modified without changing the DNA code itself.
Exploring this further, a 2015 study (Yehuda et al., 2016) assessed 32 Holocaust survivors and a total of 22 of their children, as well as a small control group consisting of 8 participants. They found that the children of the survivors of the Holocaust had epigenetic changes to a gene that was linked to their levels of cortisol, a hormone involved in the stress response.
Such a finding was extraordinary and has major implications; it would mean that experiences throughout your lifetime, particularly traumatic ones, would influence biological changes in your offspring for generations to come.
However, the study by Yehuda and colleagues was quickly criticized for a number of reasons. Firstly, the scientists studied blood, which is a mixture of cell types, making it difficult to ascertain the cause for the changes reported by participants. Secondly, the study only looked at a small subset of genes- a proverbial drop in the ocean of the human genome. Without studying several generations and searching more widely in the genome, we can’t conclusively say any changes were due to epigenetic inheritance.
While initially exciting, we must be cautious when interpreting evidence for epigenetics. The concept of our environment altering the expression of our genes is a well-established, evidence-based theory. Conversely, evidence for this process happening between generations is lacking and methodological flaws in research haven’t helped the cause.
This doesn’t necessarily mean we should rule out the epigenetic heritability of trauma entirely. Taking a slightly more empirical approach, controlled experiments have used mice to explore the intergenerational effect of trauma (Dias & Ressler, 2013). By taking this approach, researchers can control the environment and the breeding of the mice.
The study by Dias and Ressler showed an intergeneration effect of trauma associated with scent (Dias & Ressler, 2013). Seemingly channeling B.F Skinner and his infamous ‘Skinner Box’, researchers blew acetophenone (scent of cherry blossom) through the cages of adult male mice, zapping their feet with an electric current at the same time. Repeating this procedure several times, the mice associated the smell with pain.
Once the male mice had been conditioned to associate the scent with pain, they bred with female mice. Amazingly, when their offspring smelled the scent of acetophenone, they showed a more fearful response than offspring from mice who hadn’t been conditioned to fear the scent.
How we do know they hadn’t simply learned about the smell from their parents? They were raised by unrelated mice who had never smelled the scent. Yet another generation later, the ‘grandchildren’ of the original mice also showed heightened sensitivity to the scent.
Much like Kellerman’s observation that offspring were more sensitive to trauma, the second and third generation of mice did not appear to fear the scent itself, but rather had a heightened sensitivity to it. This is important as the offspring don’t always display the same trait as their ancestor developed.
Such findings are extremely exciting, albeit extremely rare, especially when studying mammals, including mice. The prospect of understanding how human attributes can emerge from our experience, or more tantalizingly, our ancestors, would be revolutionary and would have major implications for psychology, as well as numerous other medical fields. These mechanisms still aren’t well-understood but given time and patience, we may yet come to understand how our life experiences can shape the lives of our descendants.
References
Dias, B. G., & Ressler, K. J. (2013). Parental olfactory experience influences behaviour and neural structure in subsequent generations. Nature Neuroscience, 17, 89-96.
Kellerman, N. P. (2013). Epigenetic transmission of Holocaust trauma: Can nightmares be inherited? The Israel Journal of Psychiatry and Related Sciences, 50(1), 33-39.
Yehuda, R., Daskalakis, N. P., Bierer, L. M., Bader, H. N., Klengel, T., Holsboer, F., & Binder, E. B. (2016). Holocaust exposure induced intergenerational effects on FKBP5 methylation. Biological Psychiatry, 80(5), 372-380.