Can Researchers Include Participants With Impaired Decision-Making Capacity?

Can someone participate in research if they cannot make the research decision entirely on their own?

Some research questions cannot be answered without studying conditions that may themselves affect decision-making. Dementia, delirium, serious neurological disorders, intellectual disabilities, brain injuries, severe psychiatric conditions, and acute illness can sometimes impair a person's ability to understand or evaluate a research decision.

That creates an ethical tension. Excluding everyone with impaired decision-making capacity may appear protective, but it can make important conditions difficult or impossible to study. Including people without adequate safeguards can expose individuals who cannot fully protect their own interests to inappropriate risk.

The question is therefore not simply whether impaired capacity permits or prohibits participation. Researchers need to determine whether inclusion is scientifically and ethically justified, how capacity will be assessed, who can authorize participation when necessary, and how the participant's own wishes will continue to matter.

Impaired capacity does not automatically prohibit research participation

Impaired capacity changes the consent problem, but it does not erase the participant

A diagnosis is not a capacity assessment

A person may have a condition associated with cognitive impairment without necessarily lacking the ability to make a particular research decision. Conversely, someone without a longstanding diagnosis may temporarily lose decision-making capacity because of acute illness, delirium, medication, injury, or another circumstance.

Capacity may also fluctuate or vary with the complexity of the decision. Researchers should therefore avoid using diagnostic categories as automatic substitutes for assessing the ability that actually matters.

This follows the broader distinction between being vulnerable and being unable to provide informed consent. Impaired decision-making capacity is one particular source of vulnerability, not a synonym for vulnerability generally.

Capacity concerns whether the person can make the relevant research decision

Research-consent capacity generally concerns abilities needed to understand and evaluate relevant information and make a choice about participation. Exactly how capacity should be assessed depends on the research setting, applicable law, institutional requirements, and the participant's circumstances.

A complex interventional study involving substantial uncertainty may place different decision-making demands on a participant than a brief minimal-risk interview. That does not mean low-risk research has no consent standard. It means capacity is considered in relation to the actual decision rather than as an abstract measure of intelligence.

Researchers should support decision-making before replacing it

When a prospective participant appears to have difficulty understanding a study, the first response need not always be to transfer the decision to someone else.

SACHRP's recommendations regarding research involving people with impaired decision-making capacity identify several approaches that may support decision-making, including involving a trusted person during the consent process, allowing additional time, and taking steps to improve or enhance decision-making when the impairment may be amenable to intervention. These recommendations are advisory rather than binding regulations.

The principle is important: assistance with decision-making and substitution of decision-making are not the same intervention.

Impaired capacity can be temporary, progressive, or fluctuating

A participant may lack capacity today and regain it later. Another may enter a longitudinal study with capacity and gradually lose it. Someone else may fluctuate between periods of greater and lesser ability.

That creates practical questions that should ideally be anticipated in the protocol. Will capacity be reassessed? What happens if it is lost during the study? If it returns, will the participant be asked to provide their own consent? How will previously expressed preferences be handled?

SACHRP has recommended planning for the possibility that a participant who initially provides consent may later lose the ability to assess their own needs and interests, including circumstances in which a designated representative may need to assume a legally authorized role if permitted.

A legally authorized representative is defined by applicable law

Under the U.S. Common Rule, a legally authorized representative is an individual, judicial body, or other body authorized under applicable law to consent on behalf of a prospective participant to participation in the procedures involved in the research. Where applicable law does not address the issue, the regulatory definition may also look to institutional policy consistent with the law of the jurisdiction.

This means researchers cannot simply nominate a convenient relative and assume that person's signature constitutes legally effective permission. Who may act on behalf of a participant depends on the governing legal and regulatory framework.

Representative authorization does not make the participant ethically invisible

A participant who cannot provide legally effective informed consent may still understand parts of the research, express preferences, ask questions, cooperate willingly, or clearly resist participation.

SACHRP recommends seeking the assent of participants who lack consent capacity when they are capable of providing it and giving careful attention to dissent, including nonverbal communication that indicates unwillingness to participate. It also recommends encouraging participants, where possible, to express opinions about the nature and extent of their research participation.

The precise legal status and requirements for assent or dissent vary by jurisdiction and research framework. Ethically, however, impaired capacity should not be mistaken for absence of preferences.

Including people who lack capacity requires scientific justification

A crucial question is whether the research actually needs to involve participants who cannot consent independently.

CIOMS guidance generally emphasizes that research involving people who are unable to give informed consent should be responsive to conditions affecting such individuals and that comparable research should not be feasible with people capable of providing informed consent when additional protections for incapacity are invoked.

This reflects an important principle: impaired capacity should not become a convenient recruitment characteristic. If the scientific question can be answered equally well using participants who can consent independently, exposing people who cannot protect their own interests may be difficult to justify.

Risk becomes especially important when participants cannot authorize exposure themselves

Representative permission does not give researchers unlimited ethical permission to expose someone to research risk.

The acceptability of research involving people who lack consent capacity depends on the governing ethical and regulatory framework, including whether the research offers a prospect of direct benefit and the level of risk when it does not.

International guidelines place particular restrictions on research involving people unable to consent when there is no prospect of direct benefit. Researchers should therefore verify the exact standards applicable to the study rather than infer a universal numerical threshold.

Excluding people with impaired capacity can also distort the evidence

There is another side to the problem. If people with dementia are routinely excluded from dementia research because dementia can impair capacity, or people experiencing severe forms of a condition are excluded because they are more difficult to consent, evidence may become concentrated among people with milder disease.

The resulting findings may then be least informative for those most affected by the condition.

This is why excluding vulnerable participants can itself raise ethical concerns. Protection should reduce inappropriate exposure without making entire populations scientifically invisible.

Capacity can change during a longitudinal study

The ethical advantage comes from anticipating changing capacity rather than discovering halfway through the study that the consent strategy assumed capacity would remain static.

Impaired capacity does not reduce research ethics to obtaining someone else's signature

Plan for capacity rather than discovering it at the consent table

If impaired or changing capacity is foreseeable, address it when designing the protocol. Waiting until recruitment begins leaves investigators making difficult ethical and legal decisions ad hoc.

These measures belong within the broader framework of additional protections for participants who may be vulnerable. The objective is not merely to obtain legally valid authorization. It is to conduct research without unnecessarily removing the participant's remaining ability to understand, choose, communicate, and object.

Before enrolling participants with possible impaired capacity, check the full decision pathway

Frequently asked questions about impaired decision-making capacity in research

Impaired capacity changes who authorizes research, not whether the participant still matters

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