Explore the key biometric cues that reveal how our bodies react automatically to stress, fear, anger, and other strong emotions, enabling tactical operators to detect potential threats through physiological changes.

When a person undergoes pressure, stress, a threatening situation, or a strong emotional response, their body undergoes a series of physiological changes mostly controlled by the Autonomic Nervous System (ANS) and triggered by hormones like adrenaline. These automatic responses form the basis of lie detector tests (polygraphs). The involuntary, observable physiological reactions are called biometric cues.

Because these cues are closely linked to the fight-or-flight response, tactical operators can observe and interpret them as early signs that someone might be facing a potential threat.

Why Are Biometric Indicators Important?

Simply put, biometric indicators occur due to emotional changes. People experience emotions when something happens, is about to happen, or even when they think about something that might affect them. Emotions prompt rapid physiological reactions that help the body respond effectively to the cause of the feeling.

Baselines and Anomalies of Biometric Indicators

The body always tries to maintain balance — a “baseline” — such as normal heart rate, breathing, temperature, and blood pressure. When situations or emotions change, the body reacts and tries to adapt to the new condition. This balance is the baseline for biometric indicators. Any noticeable changes from this baseline serve as initial signs of anomalies.

When establishing a baseline in any domain, the tactical observer must understand the context because this determines what is normal and what is abnormal. For example, a person shaking might be an anomaly—shaking is an autonomic nervous system response caused by blood being diverted from limbs to vital organs and muscles in preparation for fight or flight. But shaking can also mean the person is cold or nervous. So, it’s crucial to determine if the shaking is due to environment or physiological emotion.

The key question for anomaly detection is: Does the person’s behavior fit the situation? Once the baseline is set, anomalies become easier to spot.

Three Types of Biometric Anomalies:

1. Biometric cues that do not fit the situation.
2. Sudden or gradual changes in observable biometric cues; sudden changes strongly indicate emotional shifts.
3. Cues that a person may fake or hide, such as calming behaviors meant to control or mask true feelings.

Emotions and Their Biometric Cues

Anger

Paul Ekman famously said, “The face of attack, of violence, is anger.”

Anger often arises when a person feels physically or psychologically restrained, insulted, or harmed. Evolutionarily, anger helped humans survive by motivating fight or resistance to threats. Anger triggers strong physiological signals like a flushed face (redness), flared nostrils, increased heart rate and breathing, and visible tension. These signals clearly communicate anger to others.

Fear

Fear is closely related to anger and occurs in response to physical or psychological threats. It triggers the flight response, especially when fighting or freezing won’t help. Fear’s physiological signs include rapid heartbeat, pale skin (vasoconstriction), rapid breathing, pupil dilation, sweating, and increased blood pressure. Anxiety, linked to fear, causes pupil constriction and breathing irregularities.

Contempt

Contempt often accompanies anger toward the object of disdain, sometimes leading to violence.

Disgust

Disgust shares many physiological traits with contempt, including rapid breathing, high heart rate, and increased salivation.

Our bodies give these signals involuntarily because emotions are spontaneous. Some physiological changes linked to emotions include blood pressure spikes, increased heart rate, breathing changes, salivation, sweating, muscle contractions, tremors, and eye blinking. Sometimes, when someone tries to hide or suppress emotions (masking), this can itself be a strong indicator to a skilled observer.

Eyes: The Window to Emotional State

Though the eye doesn’t reveal all emotions, it responds instantly to stress and pressure. Biometric eye cues include pupil dilation/constriction, blinking rate, and tunnel vision.

Pupil Dilation/Constriction:

Pupils normally range from 2 to 5 mm but can constrict to 1.5 mm or dilate up to 9 mm within 0.2 seconds in response to stimuli. Studies show pupils dilate when viewing positive or enjoyable things, and constrict when viewing negative or anxiety-inducing stimuli. Pupil changes also relate to anger, fear, and focusing on something specific or painful.

Blinking:

A normal person blinks 6–10 times per minute. Increased blinking indicates stress, pressure, emotional arousal, or efforts to hide feelings. For example, President Clinton blinked about 60 times per minute under intense questioning, four times his usual rate. Decreased blinking can indicate intense focus. Monitoring blink rates can help detect emotional states.

Tunnel Vision:

Tunnel vision means loss of peripheral vision, often due to intense stress or high heart rates. It narrows the visual field, making a person focus only on what’s directly ahead, ignoring surroundings. While not visible externally, a tactical observer can infer tunnel vision by the person’s lack of awareness of their environment and overly focused gaze.

Understanding these biometric indicators equips tactical professionals to read emotional and physiological states, enhancing threat detection and decision-making.