Biological Explanations for Mood Disorders

Mood disorders have strong biological influences, but they do not come from a single cause. Current research supports a multi-factor view: genetic vulnerability + brain and body systems involved in emotion + life stressors and learning history.

Genetics and heritability

Mood disorders tend to cluster in families, and twin studies suggest a meaningful genetic contribution—especially for bipolar disorders.

Major depressive disorder (MDD): Heritability estimates are often in the moderate range (approximately 30–40%), meaning many people inherit some genetic risk, but genes alone do not determine outcome.

Bipolar disorders: Heritability estimates are typically higher (often reported around 60–85% in twin-study syntheses), suggesting a stronger genetic component than in unipolar depression.

However, no single “depression gene” has been identified. Instead, depression appears to involve many genes, each contributing small effects—a pattern called polygenic inheritance. Large genome-wide association studies have identified numerous genetic variants associated with depression risk, but each individual variant explains only a tiny fraction of overall risk.

Neurotransmitters: Beyond the “Chemical Imbalance” Myth

Many medications for depression affect neurotransmitters such as serotonin and norepinephrine, which are involved in sleep, appetite, arousal, and mood regulation. For decades, this led to the popular belief that depression is caused by a “chemical imbalance”—specifically, low serotonin levels. However, this explanation is now considered an oversimplification.

A widely discussed umbrella review examined decades of research across multiple areas—serotonin levels in body fluids, receptor binding studies, genetic studies, and tryptophan depletion experiments—and found no consistent evidence that depression is caused by low serotonin specifically (Moncrieff et al., 2022). The researchers concluded that the “chemical imbalance” explanation, while influential, is not well-supported by the scientific evidence.

This does not mean that serotonin plays no role in brain function or that antidepressants don’t work. Critics of the umbrella review have argued that serotonin systems are still involved in depression and treatment response, just not in the simple way the “chemical imbalance” model suggests (Jauhar et al., 2023). The biology of depression is far more complex than the level of any single neurotransmitter. Current thinking emphasizes that antidepressants likely work through multiple mechanisms, including effects on neuroplasticity and neural circuit function, rather than simply “correcting” a chemical deficit.

Depression in the Brain

Depression is linked to abnormal activity in several brain regions, particularly those important for assessing the emotional significance of stimuli (the amygdala) and for regulating and controlling emotions (the prefrontal cortex, or PFC).

The Amygdala

Individuals with depression consistently show elevated amygdala activity, especially when presented with negative emotional stimuli such as photos of sad faces. This heightened reactivity occurs even when stimuli are presented outside of conscious awareness, and it persists even after negative stimuli are no longer present. This pattern suggests that depressed individuals may be more prone to react to emotionally negative information and have difficulty disengaging from it.

The Prefrontal Cortex

Individuals with depression often exhibit reduced activation in parts of the prefrontal cortex, particularly on the left side. Because the PFC normally helps regulate amygdala activation—allowing us to suppress or modulate negative emotions—decreased PFC activity may impair this regulatory function. A comprehensive review of both human neuroimaging and animal studies confirmed that prefrontal abnormalities, particularly in areas involved in reward processing and emotional regulation, are among the most consistent findings in depression research (Pizzagalli & Roberts, 2022).

Recent neuroimaging research also highlight the importance of connectivity between brain regions, not just activity within individual regions. Studies show that the functional connections between the amygdala and various prefrontal regions are altered in depression and may normalize with successful treatment.

 

 

Depression is linked to abnormal activity in several regions of the brain (Fitzgerald, Laird, Maller, & Daskalakis, 2008) including those important in assessing the emotional significance of stimuli and experiencing emotions (amygdala), and in regulating and controlling emotions (like the prefrontal cortex, or PFC) (LeMoult, Castonguay, Joormann, & McAleavey, 2013).

Individuals with depression show elevated amygdala activity (Drevets, Bogers, & Raichle, 2002), especially when presented with negative emotional stimuli, such as photos of sad faces (Surguladze et al., 2005). Interestingly, heightened amygdala activation to negative emotional stimuli among depressed persons occurs even when stimuli are presented outside of conscious awareness (Victor, Furey, Fromm, Öhman, & Drevets, 2010), and it persists even after the negative emotional stimuli are no longer present (Siegle, Thompson, Carter, Steinhauer, & Thase, 2007).

Additionally, individuals with depression exhibit less activation in the prefrontal, particularly on the left side (Davidson, Pizzagalli, & Nitschke, 2009). Because the PFC can dampen amygdala activation, thereby enabling one to suppress negative emotions (Phan et al., 2005), decreased activation in certain regions of the PFC may inhibit its ability to override negative emotions that might then lead to more negative mood states (Davidson et al., 2009). These findings suggest that depressed persons are more prone to react to emotionally negative stimuli, yet have greater difficulty controlling these reactions.

Depression and Cortisol

Since the 1950s, researchers have noted that depressed individuals often have abnormal levels of cortisol, a stress hormone released by the neuroendocrine system during times of stress. Many people with depression show elevated cortisol levels, especially those reporting a history of early life trauma such as loss of a parent or childhood abuse.

High cortisol levels appear to be both a consequence and a potential cause of depression. Elevated cortisol is a risk factor for future depression, and cortisol activates the amygdala while reducing activity in the prefrontal cortex—both patterns associated with depressive states. Because stress leads to increased cortisol release, chronic stress may precipitate depression in part through these hormonal and brain changes.

The Diathesis-Stress Model

Research consistently supports that stressful life events can trigger depression. Significant losses—such as death of a loved one, divorce, and serious health or financial problems—often precede depressive episodes. Exit events, in which an important person departs (through death, divorce, or moving away), are particularly likely to trigger depression, especially when they involve humiliation or devaluation. For example, people whose romantic partner initiates a breakup develop major depression at more than twice the rate of people who experience the death of a loved one.

Childhood adversity also matters: individuals exposed to traumatic stress during childhood—including separation from a parent, family turmoil, and physical or sexual abuse—face heightened risk of developing depression at any point in their lives. A review of 16 studies involving over 23,000 participants found that those who experienced childhood maltreatment were more than twice as likely to develop recurring and persistent depression (Nanni et al., 2012).

Of course, not everyone who experiences stress or childhood adversity develops depression—most do not. This observation supports a diathesis-stress interpretation: certain predispositions or vulnerability factors influence how people react to stress. But what are these predispositions?

Gene-Environment Interactions: A Cautionary Tale

In 2003, a landmark study by Caspi and colleagues reported that people who experienced stressful life events were more likely to develop depression if they carried one or two short versions of the 5-HTTLPR gene (which affects serotonin transporter function) compared to those with two long versions. This finding generated enormous excitement about gene-environment interactions in psychiatry.

However, attempts to replicate this finding have yielded mixed results. A large collaborative meta-analysis combining data from over 38,000 participants across 31 studies found no strong evidence for this specific gene-environment interaction (Culverhouse et al., 2018). While some meta-analyses have found small effects, the overall picture suggests that if the interaction exists, it is much smaller than originally reported and may depend on specific conditions that are difficult to replicate.

This does not mean that genetic vulnerability to stress is unimportant—it likely is. Rather, it illustrates that identifying specific genes involved in complex traits like depression is extremely challenging. Depression’s genetic architecture appears to involve thousands of genetic variants, each with tiny effects, rather than a few genes with large, easily detectable effects.

Cognitive Theories of Depression

Cognitive theories propose that depression is triggered by negative thoughts, interpretations, self-evaluations, and expectations. These theories represent another form of diathesis-stress model, in which the diathesis is cognitive vulnerability rather than (or in addition to) biological vulnerability.

Beck’s Cognitive Theory and Depressive Schemas

Psychiatrist Aaron Beck proposed that depression-prone individuals possess depressive schemas—mental predispositions to think about most things negatively. These schemas contain themes of loss, failure, rejection, worthlessness, and inadequacy. They may develop early in childhood in response to adverse experiences, then remain dormant until activated by stressful or negative life events.

Once activated, depressive schemas prompt dysfunctional and pessimistic thoughts about the self, the world, and the future—what Beck called the “cognitive triad.” This thinking style is maintained by cognitive biases: systematic errors in processing information that lead people to focus on negative aspects of experiences, interpret ambiguous situations negatively, and have difficulty recalling positive memories.

For example, a person with a depressive schema centered on rejection might be hypervigilant to social cues suggesting rejection (such as another person’s frown), interpret such cues as definitive signs of rejection, and automatically recall past rejection experiences. Longitudinal studies have supported Beck’s theory by showing that this negative, self-defeating thinking style—when combined with life stress—predicts the later onset of depression.

Hopelessness Theory

Hopelessness theory proposes that a particular style of explaining negative events leads to hopelessness, which in turn leads to depression. Hopelessness involves expecting that bad outcomes will occur (or desired outcomes won’t occur) and believing there is nothing one can do about it.

According to this theory, people who explain negative life events using stable (“It’s never going to change”) and global (“It’s going to affect my whole life”) causes are at greater risk for depression than those who use unstable (“It’s fixable”) and specific (“It applies only to this situation”) explanations.

Consider a student who does poorly on a law school admissions test. A hopelessness-promoting explanation would be: “I lack intelligence, and it’s going to prevent me from ever finding a meaningful career.” A more adaptive explanation would be: “I was sick that day, so my low score was a fluke.” Research supports this theory: in one study, people with a tendency to make negative inferences about bad life events were seven times more likely to develop depression over the following six months compared to those without this cognitive style.

Rumination

Rumination is the repetitive, passive focus on one’s depressed feelings and symptoms, rather than actively problem-solving or distracting oneself. When people ruminate, they dwell on questions like “Why am I so unmotivated?” or “What’s wrong with me?” without taking action to address these concerns.

Rumination was first studied as a potential explanation for why women experience depression at higher rates than men. Research has confirmed that women are more likely than men to ruminate when sad or depressed. The tendency to ruminate is associated with more severe depression symptoms, higher risk of major depressive episodes, and longer-lasting episodes. Importantly, rumination appears to be a modifiable risk factor—cognitive therapies that teach people to recognize and interrupt ruminative thinking patterns can help prevent and treat depression.