Hormones influence human behaviour.
They can affect mood, stress responses, sleep, irritability, motivation and the way the brain processes emotional information.
That is well established.
What is considerably harder to justify is the next step: assuming that because hormones influenced someone’s behaviour, responsibility for that behaviour should automatically be reduced or removed.
Recent research across psychology, endocrinology and forensic psychiatry suggests these are two very different questions.
A 2026 review of menstrual-cycle-related mental health research found that hormonal fluctuations can produce clinically significant psychological symptoms in a minority of people, particularly those with conditions such as premenstrual dysphoric disorder.
But the same review emphasised something equally important: for most people, ordinary hormonal fluctuations do not produce clinically significant changes in mood or psychological functioning.
Other research examining testosterone and aggression has reached a similarly cautious conclusion. Hormones can be associated with behavioural tendencies, but the relationships are generally small, highly dependent on context and far from deterministic.
For institutions deciding questions of responsibility, this distinction matters.
A biological influence can help explain behaviour without becoming a verdict on accountability.
Hormones are part of almost every human decision
There is nothing unusual about behaviour having a biological explanation.
Every decision humans make ultimately occurs inside a biological system.
Cortisol changes during stress.
Testosterone responds to competition and social challenges.
Estradiol and progesterone fluctuate during the menstrual cycle.
Hormonal systems also change during puberty, pregnancy, the postpartum period and menopause.
These chemicals interact with neurotransmitters, the autonomic nervous system and brain regions involved in emotion and decision-making.
Finding that hormones contributed to a particular emotional state is therefore scientifically meaningful.
But if the existence of a biological influence were enough to remove accountability, the principle could quickly become difficult to apply consistently.
Stress hormones influence an angry employee.
Testosterone may influence competitive behaviour.
Sleep deprivation changes cortisol and decision-making.
Alcohol alters neurochemistry.
Fear changes adrenaline.
Human behaviour almost always has a biological component.
The important question is therefore not whether biology was involved.
It is what that biology actually did to the person’s capacity to understand and regulate their behaviour.
Most hormonal fluctuations do not cause severe impairment
This becomes especially important when discussing menstrual-cycle hormones.
A 2026 clinical review by Ellen Lambert, Louise Nolan and Katja Schmalenberger examined the strongest available evidence on hormone sensitivity, premenstrual dysphoric disorder and premenstrual exacerbation.
The researchers emphasised that ordinary menstrual-cycle fluctuations can produce mild changes in mood, energy or cognition.
But for most menstruating individuals, these changes remain manageable and do not interfere substantially with daily functioning.
A smaller group experiences much more severe hormone-sensitive symptoms.
Premenstrual dysphoric disorder, or PMDD, affects an estimated 3% to 8% of menstruating individuals and can involve severe depression, irritability, anxiety and other symptoms during particular phases of the cycle.
This is a legitimate medical condition.
But recognising that condition does not mean assuming that every hormonal fluctuation produces the same effect in every person.
That would replace one scientific mistake with another.
Even severe hormonal sensitivity does not produce one predictable cognitive effect
A systematic review published in the Journal of Affective Disorders examined cognition and behaviour across the menstrual cycle among people with PMDD.
The researchers reviewed 21 papers containing 94 outcome measures and a combined sample of 1,222 participants.
Despite PMDD being a recognised disorder involving severe cyclical symptoms, evidence that objective cognitive performance changed consistently across the menstrual cycle was limited and inconsistent.
Some psychological processes appeared to change.
Negative attentional bias, emotional processing and behavioural experiences may be particularly relevant.
But the evidence did not support a simple model in which hormonal change predictably switches normal reasoning or control off.
This matters for accountability.
A diagnosis or biological mechanism does not tell us exactly what one individual was capable of understanding or controlling at one particular moment.
Testosterone provides an even clearer example
Testosterone has probably suffered more from biological stereotypes than almost any other hormone.
It is frequently described as though more testosterone automatically produces more aggression.
The research is considerably less dramatic.
A major meta-analysis examined evidence from baseline testosterone measurements, changes in testosterone and experiments in which testosterone was manipulated.
Baseline testosterone had only a very weak association with aggression.
Changes in testosterone were also associated with aggression, but the relationships remained small and varied substantially between groups and circumstances.
Most importantly, when researchers looked specifically at studies attempting to manipulate testosterone experimentally, the estimated causal effect on human aggression was small and not statistically significant.
In other words, testosterone may contribute to behaviour.
It does not function like an aggression switch.
Hormones change probabilities, not necessarily choices
This provides a useful way to interpret biological behavioural research.
A hormone may change the probability that someone feels irritable.
It may alter sensitivity to social threats.
It may make certain emotional responses stronger or weaker.
None of those findings necessarily mean the individual loses the ability to choose between different actions.
Consider anger.
Two people can experience the same intense anger and respond very differently.
One may shout.
Another may leave the room.
A third may say nothing.
A fourth may become violent.
The emotional state influences the decision environment.
It does not automatically determine the decision.
Hormonal explanations should generally be interpreted in the same way unless there is evidence that the associated condition produced a much more profound impairment.
This is where medicine and accountability separate
Medicine and law are often asked to evaluate the same person, but they are answering different questions.
A clinician may ask:
What biological or psychiatric process contributed to this behaviour?
The justice system asks something closer to:
What was this person capable of understanding and controlling at the time?
A 2026 paper in the International Journal of Law and Psychiatry argues that psychiatric diagnoses and legal findings of criminal unaccountability are fundamentally different constructs.
Linda Gröning and Kenneth Hugdahl warn against assuming that a diagnosis, symptom or functional impairment maps neatly onto a legal conclusion.
People with the same diagnosis can experience very different symptoms.
Symptoms of similar severity can affect different abilities.
And the same person can function differently at different times.
The implication extends naturally to hormonal evidence.
A blood test showing a particular hormone level can establish biology.
It cannot independently establish legal incapacity.
A hormone test cannot tell a court whether someone knew something was wrong
Imagine that a laboratory establishes that someone was experiencing an unusual hormonal state at the time of an incident.
That may be valuable evidence.
But the laboratory result cannot answer several questions that are central to responsibility.
Did the person understand what they were doing?
Did they appreciate the likely consequences?
Did they recognise that their behaviour was wrong?
Could they choose between alternative actions?
Did they plan what happened?
Did they attempt to conceal it afterwards?
These questions concern functioning and behaviour, not hormone concentration alone.
Biology may contribute to the explanation.
The wider evidence determines what the explanation means.
South African law already reflects this distinction
South African criminal law provides a useful example of how this principle can operate.
Section 78 of the Criminal Procedure Act deals with mental illness or intellectual disability and criminal responsibility.
The law does not say that the existence of a medical condition automatically removes responsibility.
The relevant question is whether the condition made the person incapable of appreciating the wrongfulness of the act or acting in accordance with that appreciation.
This is a functional test.
The diagnosis provides part of the evidence.
The effect of the diagnosis on capacity determines its legal significance.
The Act also recognises an intermediate position.
If a person remained criminally responsible but their capacity was diminished by mental illness or intellectual disability, the court may take that reduced capacity into account during sentencing.
Responsibility is therefore not necessarily treated as an all-or-nothing concept.
This provides a useful model for hormonal evidence
The same logic can be applied when hormonal changes contribute to a medical or psychological condition.
There are at least three very different situations.
In the first, hormone fluctuations are present but have little meaningful effect on functioning.
In the second, they contribute to substantial emotional or psychological symptoms while the person retains meaningful decision-making capacity.
In the third, a hormone-sensitive psychiatric condition could become so severe that the person’s ability to understand reality or regulate behaviour is profoundly impaired.
Those situations should not automatically produce the same conclusion.
A fair system needs evidence about the individual rather than assumptions about the hormone.
There is also a danger in removing accountability too easily
Overstating hormonal effects creates another governance problem.
It risks converting biological differences into stereotypes about entire groups of people.
If ordinary menstrual fluctuations were treated as evidence that women have less control over their decisions, the argument would extend far beyond criminal justice.
It could be used to question women’s reliability in leadership, employment, finance, politics or other positions requiring judgement.
The science does not support that conclusion.
The 2026 clinical review explicitly notes that most naturally cycling individuals do not experience clinically significant psychological impairment from normal hormonal fluctuations.
Recognising genuine hormone-sensitive disorders therefore requires precision.
Otherwise, an attempt to acknowledge women’s health could inadvertently revive the much older stereotype that women’s decisions are governed by their hormones.
The same standard should apply to men
Consistency matters here.
If hormonal explanations are considered relevant to accountability, they cannot logically be treated as a uniquely female phenomenon.
Men also experience hormonal variation.
Testosterone changes in response to competition and social environments.
Cortisol changes under stress.
Sleep, illness, medication, age and numerous other factors influence endocrine function.
Yet society does not generally assume that a man who experiences elevated testosterone is therefore unable to take responsibility for an aggressive decision.
The research on testosterone supports that caution.
Its relationship with aggression is real enough to study but far too weak and context-dependent to determine an individual’s behaviour by itself.
A scientifically consistent accountability framework should apply the same principle regardless of sex.
Real impairment should still matter
None of this means hormonal disorders should be dismissed.
PMDD, postpartum psychiatric illnesses and severe hormone-sensitive mood disorders can cause profound suffering and require appropriate medical treatment.
In an individual case, such a condition may also be relevant when assessing someone’s mental functioning.
The key is not to move directly from biological explanation to legal conclusion.
A credible assessment should establish the condition, document its severity, examine its timing and determine what effect it actually had on the person’s functional capacity.
This protects people with genuine impairment while avoiding the assumption that everyone experiencing the same hormonal process loses the same degree of control.
Scientific evidence should inform responsibility, not replace it
As neuroscience and endocrinology become increasingly sophisticated, courts and other institutions will encounter more biological explanations for human behaviour.
The temptation will be to treat measurable biology as especially objective.
A hormone concentration looks more concrete than someone’s account of their emotional state.
But objective measurement does not automatically make the variable decisive.
A blood test can accurately tell us how much of a hormone was present.
The difficult question is what that concentration meant for this person’s behaviour in this situation.
That requires clinical interpretation, behavioural evidence and, where legal responsibility is concerned, a legal standard.
Accountability and compassion can exist together
Perhaps the most important mistake is treating accountability and medical understanding as opposites.
A person can deserve treatment and still be required to answer for their behaviour.
A biological condition can justify mitigation without necessarily eliminating responsibility.
Serious impairment can be recognised without converting every hormonal fluctuation into diminished capacity.
Good governance should be capable of holding all of these ideas at the same time.
Hormones influence us.
Sometimes that influence is clinically significant.
Occasionally it may contribute to profound psychiatric impairment.
But the existence of a biological explanation is not, by itself, evidence that human agency disappeared.
The scientifically defensible question is therefore not simply, Were hormones involved?
They almost certainly were.
The more important question is:
What could the individual still understand, choose and control?
That is where an explanation of behaviour becomes a meaningful assessment of responsibility.
Source Information
Primary Clinical Review: Gold-standard evidence and best practice guidance for menstrual cycle-informed clinical care: An overview for clinicians
Authors: Ellen R. Lambert, Louise N. Nolan and Katja M. Schmalenberger
Journal: British Journal of Clinical Psychology
Published: 20 May 2026
DOI: 10.1111/bjc.70066
Systematic Review: Cognition and behaviour across the menstrual cycle in individuals with premenstrual dysphoric disorder
Authors: Audrey Henderson, Maria Gardani, Gillian Dyker and Lynsay Matthews
Journal: Journal of Affective Disorders
Volume: 371
Pages: 134–146
Published: 15 February 2025
Studies reviewed: 21
Total participants: 1,222
DOI: 10.1016/j.jad.2024.11.033
Forensic Psychiatry Research: Criminal responsibility and mental disorder: Why do heterogeneous constructs not overlap?
Authors: Linda Gröning and Kenneth Hugdahl
Journal: International Journal of Law and Psychiatry
Volume: 108
Published: 2026
Article: 102236
DOI: 10.1016/j.ijlp.2026.102236
Hormone and Behaviour Meta-analysis: Is testosterone linked to human aggression? A meta-analytic examination of the relationship between baseline, dynamic, and manipulated testosterone on human aggression
Authors: S. N. Geniole, B. M. Bird, J. S. McVittie, R. B. Purcell, J. Archer and J. M. Carré
Journal: Hormones and Behavior
Volume: 123
Article: 104644
DOI: 10.1016/j.yhbeh.2019.104644
South African legal context: Section 78 of the Criminal Procedure Act 51 of 1977, dealing with mental illness, intellectual disability and criminal responsibility.
