How Do You Decide Whether a Participant Has the Capacity to Give Informed Consent?

How do you know whether someone can make their own research decision?

A prospective participant seems confused about part of the study. Another has dementia but answers questions coherently. Someone with a psychiatric diagnosis wants to participate despite risks that concern the research team. An acutely ill patient understands some information but appears intermittently disoriented.

Which of these people has capacity to give informed consent?

There is no reliable answer based solely on diagnosis, appearance, disability, literacy, age, or whether the person's decision seems sensible to the researcher. Consent capacity concerns the person's ability to make the particular research decision after receiving appropriate information and support. In straightforward situations, that may be evident during an ordinary consent conversation. In others, a more deliberate or formal assessment may be warranted.

Assess the person's ability to make this decision, not whether they belong to a diagnostic category

Capacity is about the decision the participant is being asked to make

Consent capacity is not an all-or-nothing personal trait

It is tempting to classify people as either "having capacity" or "lacking capacity." In practice, consent capacity is more nuanced.

Recommendations from the U.S. Secretary's Advisory Committee on Human Research Protections describe consent capacity as occurring along a continuum, being task-specific, and depending on the nature and complexity of the relevant decision. They also recognize that capacity may improve, fluctuate, or worsen over time.

This means that someone may be able to make one research decision but struggle with another that is substantially more complicated. A person may also lack adequate capacity during an acute episode and later regain it.

Start with the actual abilities needed for the research decision

No single assessment framework is universally mandated for every study. A widely used approach to decision-making capacity examines whether a person can understand relevant information, appreciate how it applies to their own situation, reason about the available options, and communicate a choice.

These abilities should be considered in relation to the actual research decision rather than as abstract cognitive exercises.

Can the participant understand the relevant information?

Understanding concerns whether the person grasps the information needed for the decision. Depending on the study, this may include that the activity is research, its purpose, what participation involves, important risks and possible benefits, alternatives where relevant, voluntariness, and the ability to withdraw.

Do not test understanding by asking participants to repeat the consent form word for word. A participant can forget terminology while understanding the concept perfectly well. Conversely, someone can recite information accurately without understanding what it means.

Open-ended questions are more useful. You might ask, "Can you tell me what you would be asked to do if you join?" or "What are the main things that could happen that matter to your decision?"

Can the participant appreciate how the information applies to them?

Understanding a fact and appreciating its personal relevance are not necessarily the same thing.

A participant might accurately state that an investigational intervention has uncertain benefit but insist that uncertainty applies to everyone except them because the researcher has secretly guaranteed their recovery. Another person might accurately describe a risk but deny that the procedure could possibly affect them for reasons that substantially distort the decision.

Assessment should therefore consider whether the participant can connect relevant study information to their own circumstances.

Can the participant reason about the options?

Reasoning does not mean reaching the conclusion that the researcher would choose. It concerns whether the participant can use the relevant information to compare options in a meaningful way.

A participant might reasonably say, "The study may help me, but the benefit is uncertain, and the possibility of this side effect matters enough that I don't want to participate." Another might reach the opposite conclusion using the same information.

Both decisions can reflect adequate reasoning.

Can the participant communicate a choice?

The participant needs a way to communicate the participation decision. Speech is not the only possible method. Depending on the person's communication needs, a decision may be expressed through writing, assistive communication, supported communication, or another reliable method.

A physical or communication disability should not automatically be interpreted as impaired decision-making capacity. Researchers should first determine whether an accessibility barrier can be removed.

Support understanding before concluding that capacity is impaired

A participant who initially misunderstands the study does not necessarily lack capacity. Perhaps the consent form is overly technical. The explanation may have been rushed. The participant may need a quieter setting, hearing support, interpretation, additional time, repetition, visual aids, or simpler language.

The ethical sequence matters: communicate effectively, correct misunderstandings, and then determine whether the participant can make the decision.

For example, difficulty reading should be addressed through an accessible consent process for participants with limited literacy, not automatically classified as impaired capacity.

Most participants do not need a formal capacity test

A formal instrument for every prospective participant would usually be unnecessary and burdensome. SACHRP recommendations state that when impaired consent capacity is not anticipated, researchers can ordinarily make an informal judgment during routine interactions in the consent process.

A participant who follows the discussion, asks relevant questions, explains the important features of the study, weighs them coherently, and communicates a stable decision may provide little reason for further assessment.

More structured evaluation becomes useful when there is a genuine reason for uncertainty.

Increase the rigor of assessment when impairment is more likely or the stakes are higher

SACHRP recommends tailoring the assessment method to the study population, research risk, and likelihood that participants may have impaired consent capacity. Formal approaches may include questionnaires, structured instruments, or independent evaluators.

The intensity of assessment should therefore be proportionate rather than automatic.

Diagnosis should trigger attention when relevant, not an automatic verdict

OHRP notes that impaired capacity can arise from trauma, some forms of mental illness, dementia, and other conditions, and may be temporary, progressive, or permanent. At the same time, federal guidance does not treat diagnostic membership itself as the capacity determination.

The more precise question is whether this person's condition currently interferes with the abilities necessary for this particular research decision.

This is especially important when considering whether a cognitive or mental health condition affects consent capacity. Some people with such conditions retain full capacity, while others may need support or a more formal assessment.

Capacity can fluctuate

Consent capacity may change with delirium, medication effects, acute illness, psychiatric symptoms, fatigue, intoxication, neurological conditions, or progressive cognitive disorders. Someone capable in the morning may be substantially confused later; another person may recover capacity after an acute condition resolves.

SACHRP specifically recommends considering reassessment, redisclosure of relevant information, and reconsent when changes in capacity are anticipated or discovered during a study.

Longitudinal research should therefore ask not only whether the participant can consent at enrollment but whether capacity may change before important later procedures or decisions.

Loss of capacity does not automatically authorize just anyone to consent

If an adult lacks capacity to provide their own consent, the next step depends on the applicable law, regulations, protocol, and ethics approval. OHRP states that under HHS-regulated research, a legally authorized representative may consent when permitted under the law of the jurisdiction, unless an applicable waiver of consent has been approved.

A spouse, adult child, caregiver, or friend does not automatically acquire research-consent authority merely because they know the participant well.

The question of when another person can consent on behalf of a participant therefore requires separate legal and ethical analysis.

A participant who lacks full consent capacity may still have preferences that matter

The 2024 Declaration of Helsinki states that when a person incapable of giving free and informed consent can give assent, that assent should be sought in addition to consent from the legally authorized representative, and the person's dissent should be respected.

Capacity should therefore not be treated as a switch that makes the participant disappear from the decision. Even where representative permission is required, researchers should involve the person to the extent of their abilities and according to the applicable framework.

When an initial misunderstanding does not necessarily mean incapacity

The initial error mattered, but it did not prove incapacity. The researcher first determined whether better communication could resolve the misunderstanding, then evaluated the participant's ability to make the actual research decision.

Common mistakes when judging consent capacity

Match the capacity assessment to the participant, study, and level of uncertainty

Do not begin by asking which diagnostic categories automatically require a formal capacity test. Begin by asking how likely impaired consent capacity is in the study population, how complicated the participation decision is, what risks are involved, and what evidence the research team will need when capacity is genuinely uncertain.

The assessment should protect participants without unnecessarily removing their decision-making authority. Too little scrutiny can expose people with impaired capacity to research they did not meaningfully choose. Too much scrutiny can exclude capable people merely because they belong to a population researchers perceive as vulnerable.

Before concluding that a participant can or cannot consent

Questions about capacity to consent to research

Assess the decision-making process rather than the participant's label

How to Cite This Guide

This guide is intended to be read, shared, and used in research, teaching, and academic work. If you draw on its ideas, explanations, or other content, please acknowledge the source by citing the guide. Doing so gives appropriate credit and helps your readers locate the original resource.