Anxiety and depression symptoms do not change drastically with time. This study investigates whether the reason for the stability of the symptoms can be regulated genetically or environmentally. Anxiety and depression symptoms of nine groups of identical twins were analyzed over a few years in this study. The facts about how the symptoms changed with time and how much they differed between the twins were statistically tested. Identical twins diverged from expected symptom levels for a few years but not beyond. “By middle adult life, environmental experiences contribute substantially to stable and predictable inter-individual differences in levels of anxiety and depression.”
The stability seen in the levels of depression symptoms may be the function of a genetic reference point or set point to which the body eventually returns, once the stimulus has dwindled. A contrasting hypothesis suggests an environmental set point, which suggests that positive and negative environmental experiences balance each other by elevation and depression of symptoms. Environmental experiences may modify the DNA structure to affect neural functioning or may cause a person to make bad choices later in life, leading to psychological effects. Identical twins, being genetically identical, provide an excellent system to study exclusively about environmental influences. They also help to study the period in life when certain symptoms begin to stabilize.
* In this study, nine groups of identical twins, aged between 10.6 to 66.8 years with at least two episodes of depression and/or anxiety answered the questionnaires.
* The variance in the levels of symptoms that were shared by the twins and the variance in the levels of symptoms that differed among them were calculated.
* The differences or variance within each pair were considered together with the variance of each over the years to find possible correlations.
* The genetic set point hypothesis states that identical twins will not differ in the stable levels of depression/anxiety symptoms. The data does not support this idea.
* The environmental set point hypothesis states that differences between the stable levels of twins increase linearly over time. It has been found that this idea fits the data better.
* The researchers predicted that the differences in stability points of twins will not increase indefinitely, but will plateau after middle age. The data supports this idea the best.
The conclusions reached are based on studying individuals from a wide age group and various countries. The study does not explain how the environment can affect the set point for moods. While identical twin studies are useful to rule out the contribution of genetic differences, they also dilute the effect of environmental differences, because both individuals in a pair often share most of their early-life environments.
The symptoms of anxiety and depression oscillate about a stable set point. This study attempts to pinpoint whether this set point is determined genetically or by the environment surrounding the individual. Identical twins have the same genetic composition, allowing assignment of all differences in their set points to their varied environment. The study shows that the set points are not just determined environmentally, but that they are not fixed. Over the years, these stability set points grow apart in a pair of twins until they cross middle age. As environmental experiences are unique to each individual, the change in the stability point is affected by new experiences continually. The environment may have a biological effect on genetic material, altering the production of certain neural proteins and thus, the depression symptoms. On the other hand, with age, an individual may remove himself/herself from the environment, causing mental instability.
For More Information:
The Impact of Environmental Experiences across the Lifespan on Symptoms of Anxiety and Depression
Publication Journal: Psychological Science
By Kenneth Kendler, MD; Lindon Eaves, PhD; Virginia Commonwealth University, Richmond, Virginia