In 2011 in Glasgow, Scotland, a fourteen-year-old boy was sentenced to seven years in jail for stabbing his thirty-four-year-old foster mother to death. Leading up to this tragedy, the boy had suffered a troubled childhood, moving around from one foster family to another. After two failed placements, he ended up with a third foster family. This one, however, only wanted one child, causing him to be separated from his half sisters. Although he was reported to have settled in well with his third foster family, the boy’s life took a turn for the worse and the result was catastrophic (“Dawn McKenzie FAI: Foster carer’s death ‘avoidable’,” 2015). What caused this boy to kill his foster mother? One possible explanation for his sudden turn to violence is the absence of maternal love and nurture in his early life, which in turn, might have provided him with the stability necessary to avoid killing his foster mother.
Adolescents who grow up in foster care because they were neglected or abused as children face more than their share of life challenges. Studies have shown that maternal neglect and abuse can lead to higher levels of depression in adolescents. Therefore, it comes as no surprise that adolescents raised in foster care are also at greater risk of seriously considering and attempting suicide than other youth (Pilowsky & Wu, 2006). Indeed, scientists have estimated that nearly two out of three suicide attempts stem from abusive or traumatic childhood experiences (Dube et al., 2001).
Why is it that children who are neglected by their mother, or otherwise experience abuse in their early years, suffer from depression, and increased likelihood of suicide? In trying to understand this question, scientists have studied rats, looking specifically at the effects of a mother’s care and nurture on her offspring (known as “pups”). These studies suggest that the level of care that a mother gives to her pups influences and can even alter the advancement of their brains. In particular, early experiences in pups’ lives can affect their behavior, especially how they respond to stress. These experiences can continue to influence the pups into their adult life and change their behavior when they become mothers.
So, how does a mother’s affection impact the brain of a pup? Scientists have asked whether individual behavioral differences arise as a product of both our genes and/or our life experiences? These two approaches are conveniently referred to as nature and nurture theories. Those on the nature side emphasize factors that one is born with, such as our DNA, or the “big book” with all of the important information to make our body’s cells function. We can inherit genes that make our brain healthier and resistant to mental diseases like depression or we can be born with genetic vulnerabilities that make it more likely for us to become depressed. Therefore, our “big DNA book” can encode information that makes us more likely to behave a certain way throughout our lives.
In contrast, those on the nurture side emphasize the importance of personal experiences, like the care we received from our mothers, in contributing to our mental health. This side argues that our environment matters more, taking into consideration how we were raised, our social relationships, and our surrounding culture. In this case, people argue that although we were all born with a big “DNA book” encoding information for our cells, our personal experiences play a more important role in determining our behaviors throughout our lives.
It turns out that there is evidence for both nature and nurture playing a role in behavior. The pages in the DNA book can be modified throughout life, such as by highlighting sentences, crossing out sentences or words, or gluing certain sections of pages together to make them unreadable, while putting earmarks on others to make them more accessible. These changes modify the way in which the book is read; so, even if we were born with a good set of genes that help our brain stay healthy, our life experiences (i.e., nurture), can still make it more likely for us to develop depression through learned social behaviors. Therefore, differences in social experiences have actually been shown to make changes to the DNA.
Surprisingly, the behavioral information we receive due to the amount of care provided by our mothers can be passed on from one generation to the next, creating a cycle of abuse and depression. It is thought that this can occur via the nature (changes in DNA) or nurture routes. When looking at the nurture route in rats, for example, if a female pup is raised by a mother that does not provide her with a lot of care, scientists have found that the pup is more likely to become a low-caring mother in the future. In this case, the lack of care from her mother influences her brain. Thus, the behavior of the mother is passed on to her pup, even though there is no evidence that the pages in the “DNA book” were changed.
On the other hand, in rats, there is also some evidence of behaviors of neglectful mothers being passed on to their pups via changes in the DNA, via the nature route. Scientists have determined that certain experiences a mother has can change her DNA, which in turn can influence the DNA of her pup (while she is pregnant). In this case, changes can be made in the “DNA book”, like we discussed earlier. Maternal care and diet, among other things, are able to cause these types of DNA changes. For more information on how DNA changes can influence an offspring, refer to episode seven in the podcast series, focusing on the effects of diet across many generations.
In 1999, two scientists, Darlene Francis and Michael Meaney, discovered evidence of rats passing on maternal care tendencies along the nurture route. They found that, in the rat population, some mothers were high-caring and others were low-caring, for no particular reason. They defined high-caring mothers as those who engaged in a lot of of pup licking and grooming as well as elevated levels of nursing. Low-caring mothers, on the other hand, spent much less time licking, grooming, and nursing their pups. They saw that the difference in amount of maternal care given influenced how the pups responded to stress. Although a little bit of stress is healthy, chronic stress can lead to depression and anxiety.
The differences that Francis and Meaney found in offspring of mothers that spent either more or less time mothering their pups did not only impact how young pups responded to stress. As adults, more nurtured pups also showed less signs of fear and responded better to stress than the offspring who were neglected. This finding is extremely important because it shows that the way in which maternal care influences the pups is long-lasting and can continue throughout the lives of rats.
Francis and Meaney (1999) also found that female rats whose mothers provided them with a lot of care as pups were more likely to spend more time caring for their offspring. Although it was clear that the behavior was passed down from one generation to the next, the question that arose was: how is this happening? To try to answer this question, scientists performed an “adoption” study with the pups of low- and high-caring mothers. In this study, pups born from high-care mothers were placed with low-care mothers and pups born from low-care mothers were placed with high-care mothers while growing up. They found that the pups born from low-caring mothers that were raised by high-caring mothers were much less fearful under stressful conditions than the offspring of high-caring pups that were raised by low-caring mothers. From this finding, they learned that differences in fear could be passed on from parent to offspring through the behavior of the mother influencing the offspring’s brain.
However, scientists have also found evidence that maternal care can change the DNA of offspring and that these changes can be passed down through multiple generations. In a 2004 study, Moshe Szyf and Michael Meaney found that offspring of high-caring mothers and offspring of low-caring mothers have differences in their DNA in a part of their brain that controls memory. Specifically, the DNA of pups raised by mothers that did not provide much care had some chapters of the “book” glued together. This, in turn, prevented the cell from reading this part of the book and making the gene that allows the offspring to cope with stress. If you glance at the figure below, you can see there are little red M’s attached to the DNA. These red tags indicate DNA methylation. DNA methylation is the addition of one or more methyl (one carbon atom attached to three hydrogen atoms) groups to DNA, modifying the function of the DNA. Typically, the more methylation that occurs, the less a gene is turned on. Methylation therefore alters the expression of the genes, allowing every cell in the body to carry all of the “pages of the book” with all of the information, but only allowing access to specific pages of information. Studies conducted after this one in 2004 have also shown that the changes made to the DNA methylation can be passed on from mothers to offspring. If you want to know more about DNA methylation, read Sam’s introductory essay in this series.
In the early 2000s when these studies were performed, most scientists thought that DNA methylation had a permanent effect on DNA, meaning that once the pages were glued together, they could not be taken back apart. However, this idea was proven wrong as more “adoption” studies were done. Pups born from mothers that provided high levels or low levels of care were placed with the opposite type of mother within twelve hours after birth. They found that the differences in methylation over the first week of life were reversed by placing the pups with the opposite type of mother. This was groundbreaking because it showed that the effects that maternal care has on DNA are not permanent.
Szyf and Meaney’s study also aimed to see if methylation patterns are reversible later in life, specifically, through the use of a drug. They found that, indeed, the rats injected with the drug had reversed methylation patterns. This means that somehow the “pages” that were glued together in the DNA book came apart when the drug was given, allowing access to those pages once again. This result was particularly important because although there can be DNA changes in offspring due to a mother’s behavior, the DNA changes are not necessarily permanent and can potentially be reversed later in life.
Although these studies were performed on rats, their results suggest important findings, which could possibly be applied to humans. The importance of these results for humans will be discussed further by Lizzy and Lisa in the next podcast in this series. However, it is clear that in rats, the level of maternal care that a mother gives to her pup can directly affect the DNA. Not only can maternal behaviors be passed on from a mother to her pups via the socially learned nurture route, but also in how the pages of the DNA book are glued together, earmarked, or highlighted. These changes can be passed onto the next generation, influencing the offspring’s “DNA book” as well.
Increased awareness of the importance of a mother’s love and nurture could help children in foster care or other low-care environments receive more attention. Hopefully, this could lead to better responses to stress, better ways to cope with depression, and lower risks of suicide for children throughout their lives. It is even possible that more care from a loving mother at a young age could have led the fourteen-year-old boy in Glasgow to deal with stress in a healthier way, preventing the tragic stabbing of his foster mother.
Baby Rats’ Postnatal Diet More Important than Prenatal Maternal Diet. Digital image. ALN Magazine. 2012. Web. http://www.alnmag.com/news/2012/08/baby-rats-postnatal-diet-more-important-prenatal-maternal-diet
Bagga, Savita. Introduction to DNA Methylation. Digital image. BioFiles. 2012. Web. http://www.sigmaaldrich.com/technical-documents/articles/biofiles/introduction-to-dna-methylation.html
Dawn McKenzie FAI: Foster carer’s death ‘avoidable’ (2015, August 12). Retrieved April 17, 2016, from http://www.bbc.com/news/uk-scotland-glasgow-west-33883044
Dube, S.R., Anda, R.F., Felitti, V.J., Chapman, D.P., Williamson, D.F., Giles, W.H. (2001). Childhood Abuse, Household Dysfunction, and the Risk of Attempted Suicide Throughout the Life Span: Findings From the Adverse Childhood Experiences Study. JAMA, 286(24), 3089-3096.
Francis, D., Diorio, J., Liu, D., Meaney, M.J. (1999). Nongenomic Transmission Across Generations of Maternal Behavior and Stress Responses in the Rat. Science, 286(5442), 1155–1158.
Pilowsky, D. J., & Wu, L.-T. (2006). Psychiatric symptoms and substance use disorders in a nationally representative sample of American adolescents involved with foster care. The Journal of Adolescent Health: Official Publication of the Society for Adolescent Medicine, 38(4), 351–358.
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