Early national data suggest that 2025 recorded fewer mass killings than recent peak years, continuing a broader post-pandemic decline in lethal violence. The number of homicides nationwide is projected to be the lowest since the FBI began tracking such data in 1960. While any loss of life remains unacceptable, this downward shift raises an important psychological question: What changed?

The answer is unlikely to be found in a single policy, personality type, or diagnostic category. Instead, the data invite us to view violence through the lens increasingly used in neuroscience and evolutionary biology—not as a switch that suddenly turns on, but as a threshold-dependent behavior.

A useful way to think about extreme violence is through a simple behavioral grammar:

Behavior = Archetype × Drive × Culture × Threshold

Often referred to as the ARCH model, this equation reflects a core insight from ethology, psychiatry, and systems neuroscience: no single factor causes violence. Violence emerges only when multiple necessary conditions converge and a threshold is crossed.

If any one component is absent, the behavior collapses. Strong grievance without cultural permission rarely leads to mass violence. Cultural hostility without energized individuals rarely produces attackers. Most crucially, high thresholds prevent action even when other elements are present.

Importantly, approaching the threshold does not mean a person will inevitably cross it. Many individuals experience intense grievance, anger, or despair without acting violently because at least one regulatory brake remains intact. Cultural inhibition,........

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