Everyone gets anxious from time to time when it comes to public speaking or job interviews. The impulse to pace back and forth, bite your fingernails, and lightly tap your foot up and down are all common symptoms of anxiety. But where does it come from? According to a recent study published in the Proceedings of the National Academy of Sciences, the brain function that triggers the onset of anxiety and depression is inherited.

In the U.S., anxiety is the most common mental illness, affecting 40 million adults age 18 and older, according to the Anxiety and Depression Association of America (ADAA). Anxiety disorders may develop from a complex set of risk factors, including genetics and brain chemistry.

Researchers at the University of Wisconsin-Madison believe anxiety disorders run in families, and that they have a biological basis like allergies or diabetes. "[Hyperactivity] of these three brain regions (the brain stem, the amygdala, and the prefrontal cortex) are inherited brain alterations that are directly linked to the later life risk to develop anxiety and depression," said Dr. Ned Kalin, senior author and chair of psychiatry at the UW School of Medicine and Public Health, in a news release about the findings.

In an effort to understand the heritability of neural systems linked to psychopathology, Kalin and his colleagues measured anxiety-related behavior by using high resolution functional and structural brain imaging in about 600 young rhesus monkeys from a large multi-generational family. The young monkeys were exposed to a mildly threatening situation similar to what a child would encounter — exposure to a stranger who does not make eye contact.

During this encounter, the researchers used positron emission tomography (PET) — an imaging method commonly used in humans — to identify brain regions in which increased activity predicted an individual’s level of anxiety. The researchers were able to examine where individual differences in brain function and anxiety-related behavior fell on the family tree, which also helped to identify the brain systems responsible for parent-to-child transmission of anxiety-related behavior.

The findings revealed that the brain circuit that was genetically linked to individual differences in early-life anxiety involved three brain regions, including the brain stem, the primitive part of the brain; the amygdala, the limbic brain fear center; and the prefrontal cortex, which is responsible for higher-level reasoning. The prefrontal cortex is only fully developed in humans and their primate cousins. Therefore, when brain circuits become hyperactive with anxiety, bigger problems can arise, leading to the onset of anxiety and depressive disorders.

Kalin believes anxiety can be both advantageous and destructive, depending on the circumstances. "Basically, we think that to a certain extent, anxiety can provide an evolutionary advantage because it helps an individual recognize and avoid danger, but when the circuits are hyperactive, it becomes a problem and can result in anxiety and depressive disorders," Kalin said.

A 2010 study, also from the University of Wisconsin-Madison, found anxious temperaments may be genetically transferred. The researchers identified brain regions that were hyperactive in the most anxious monkeys, and found that the more anxious behavior the monkeys displayed, the more active their amygdalas and hippocampi were. These findings correlate with the neuroscientists' beliefs that the amygdala is the area of the brain critical to anxious temperaments and emotions. The hippocampus was also the region most affected by genes.

These studies support the belief anxiety and anxious temperament seem to be inherited. Our genes play an influential role in shaping our brains. "Now that we know where to look, we can develop a better understanding of the molecular alterations that give rise to anxiety-related brain function,'' Kalin said.

Sources: Alexander A, Blangero J, Converse AK et al. Intergenerational neural mediators of early-life anxious temperament. PNAS. 2015.

Blangero J, Davidson RJ, Dyer TD et al. Amygdalar and hippocampal substrates of anxious temperament differ in their heritability. Nature. 2010.