PastPaperHero | Relevancy and reasons for excluding relevant evidence

Learning Outcomes

This article explains how the rules of evidence govern the qualifications and competency of witnesses in MBE-style fact patterns, including:

Identifying when a person is presumed competent to testify under the Federal Rules of Evidence and recognizing the limited circumstances in which that presumption can be rebutted.
Applying the core requirements of personal knowledge, an oath or affirmation, and the ability to communicate to typical bar-exam hypotheticals involving lay witnesses.
Evaluating competency issues for children and mentally impaired persons, and distinguishing true incompetency from mere credibility or weight-of-the-evidence concerns.
Recognizing the absolute disqualification of presiding judges and sitting jurors, and understanding the strict limits on juror testimony about deliberations before and after a verdict.
Analyzing how expert witnesses are qualified, how Daubert-style reliability screening operates under FRE 702, and how expert testimony differs from lay opinion.
Determining when state law, including Dead Man’s Statutes, governs witness competency in federal diversity cases through FRE 601, and predicting how that choice-of-law rule can affect admissibility outcomes on the exam.

MBE Syllabus

For the MBE, you are required to understand the rules governing who may testify and when otherwise relevant evidence is excluded because the witness is not properly qualified, with a focus on the following syllabus points:

General rule and presumption of witness competency (FRE 601–603)
Requirements of personal knowledge, oath or affirmation, and communication ability
Special rules for children, mentally impaired persons, judges, and jurors (FRE 605–606)
Qualification and admissibility of expert testimony, including Daubert-type reliability screening (FRE 702)
Effect of state competency law, including Dead Man’s Statutes, in federal diversity cases

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

Which of the following is NOT a general requirement for a person to be a competent witness?
1. Personal knowledge of the matter
2. Ability to communicate
3. No prior criminal record
4. Willingness to take an oath or affirmation
In federal court, a 6-year-old child may testify if:
1. The child is able to understand the duty to tell the truth and can communicate what was seen.
2. The child’s parent consents to the testimony.
3. The child has previously testified in court.
4. The child is represented by counsel.
Which statement about expert witnesses is correct?
1. Only doctors and scientists can be experts.
2. The court must find the expert is qualified and the testimony is reliable.
3. Experts may testify only about facts, not opinions.
4. Experts must have personal knowledge of the facts in every case.

Introduction

The Federal Rules of Evidence presume that every person is competent to be a witness unless a specific rule provides otherwise. However, not all relevant evidence will be admitted if the witness lacks the necessary qualifications. The rules on witness qualifications ensure that testimony is sufficiently reliable and that the fact-finder receives evidence from individuals who can observe, recall, and communicate what is relevant to the case.

Competency is largely a threshold issue. Once a witness clears that threshold, any weaknesses in perception, memory, or honesty usually go to the weight of the evidence (for the jury to evaluate) rather than to admissibility. A firmly biased witness or a witness with memory problems is ordinarily still competent; those issues are explored on cross-examination and affect whether the jury believes the witness, not whether the witness may testify at all.

Key Term: Competency
The legal capacity of a person to testify as a witness. Under the Federal Rules, every person is presumed competent unless a rule specifically disqualifies them.

General Rule of Competency (FRE 601)

Under FRE 601, all persons are presumed competent to testify. The court may exclude a witness only if:

The witness cannot meet the basic requirements of competency (personal knowledge, oath/affirmation, ability to communicate), or
A specific rule disqualifies the witness (most notably, the presiding judge and jurors in the case under FRE 605–606, or a state Dead Man’s Statute when applicable through FRE 601).

Modern evidence law has abolished most old common law disqualifications. A witness is not incompetent merely because:

They are a party to the case or have a financial interest in the outcome
They are the spouse of a party (spousal disqualification as a competency rule has been abolished in federal court; modern rules treat this as a privilege question, not competency)
They have a prior criminal conviction
They lack religious belief or belong to a particular religion
They suffer from mental illness or cognitive impairment, as long as they can meet the basic competency requirements

Those circumstances are fertile grounds for impeachment (attacking credibility), but they do not bar the person from taking the stand.

The trial judge decides competency as a preliminary question of admissibility under FRE 104(a). The judge applies a preponderance-of-the-evidence standard and may consider any relevant information, even if it would not itself be admissible at trial. If necessary, the judge decides competency outside the presence of the jury to avoid prejudice.

Basic Requirements for Witnesses

Although competency is broadly presumed, a witness must satisfy three core requirements:

Personal Knowledge
Oath or Affirmation
Ability to Communicate

These requirements are reflected mainly in FRE 602–604 and in the general competency provision of FRE 601.

Key Term: Personal Knowledge
The requirement that a witness can testify only to matters they have directly perceived—through their own senses—rather than speculation or second-hand information.

1. Personal Knowledge (FRE 602)

A witness must have personal knowledge of the matter and must testify from first-hand knowledge. In practice this means:

The witness must have perceived the event or condition (saw, heard, smelled, felt, etc.).
There must be evidence sufficient for a reasonable juror to find that the witness has such knowledge. The witness’s own testimony can usually establish this.
The witness must testify to facts, not pure speculation. Reasonable inferences from their observations are allowed; baseless guesses are not.

Examples:

A bystander who saw a car run a red light may testify that the car went through the red light.
That same witness may not testify that “the driver must have been texting” unless they actually saw the phone or can describe specific observations (e.g., the driver looking down at a phone in their hand) that support that inference.

Personal knowledge is distinct from hearsay:

A witness can have personal knowledge of what someone said (and may testify to that statement), even though the statement might be hearsay if offered for its truth.
If the statement is offered for some non-hearsay purpose (e.g., effect on the listener, notice, verbal act), the personal knowledge requirement is still satisfied even if the hearsay rule is not implicated.

A few practical points for the exam:

The personal knowledge requirement applies to lay witnesses. Expert witnesses may base opinions on facts they did not personally observe (see below).
Gaps in a witness’s memory (e.g., not remembering exact times or distances) normally do not destroy personal knowledge; they affect weight, not admissibility.
If a witness clearly lacks personal knowledge on a topic (“I have no idea, I wasn’t there”), testimony about that topic is irrelevant and inadmissible.

2. Oath or Affirmation (FRE 603)

The witness must declare that they will testify truthfully, either by oath or by affirmation, in a form designed to impress that duty on their conscience.

Key Term: Oath or Affirmation
A formal promise to testify truthfully, given in any form the court approves, designed to impress upon the witness the duty to tell the truth.

Key points:

The exact words do not matter. The court may modify the form of the oath or affirmation to the witness’s beliefs and circumstances (religious oath, solemn secular affirmation, simplified language for a child, etc.).
The core requirement is that the witness understands they must tell the truth and that lying in court is wrong.
Children and persons with limited understanding may be examined by the judge (often through simple questions such as “Is it good or bad to tell a lie?”) to ensure they appreciate the difference between truth and lies.
A witness who refuses to take an oath or affirmation is incompetent to testify.
Lack of any religious belief does not disqualify a witness; the oath requirement is ethical, not religious.

On the MBE, whenever a child or mentally impaired person is at issue, ask:

Do they understand they must tell the truth?
Are they willing to promise to do so?

If yes, the oath requirement is satisfied.

3. Ability to Communicate

The witness must be able to communicate what they perceived in a way that the fact-finder can understand. This includes:

Having sufficient ability to recall events (memory)
Being able to express those recollections verbally, in writing, or through sign language
Being able to understand questions and respond meaningfully

Physical or mental impairments (hearing loss, speech impairment, developmental delay, mental illness) do not automatically make a witness incompetent. Courts routinely accommodate such witnesses with interpreters, assistive devices, or modified questioning.

Examples:

A deaf witness who testifies through a sign language interpreter is competent.
A witness with a speech impediment may testify if the court and jury can understand them (often with repetition or clarification).
A witness with mild dementia may still be competent if they can recall and communicate relevant events and understand the duty to tell the truth.

What can render a witness incompetent on this ground is a complete inability to communicate (e.g., someone in a coma) or an inability to understand questions at even a basic level.

Key Term: Interpreter
A person appointed to translate for a witness who cannot communicate effectively in the language of the court. The interpreter must be qualified and must take an oath to make a true translation.

Under FRE 604:

An interpreter must be qualified—that is, have adequate skill in both languages and no disqualifying bias.
The interpreter must take an oath or affirmation to make a true translation.
The interpreter may be examined and impeached like any other witness (e.g., on bias or competency).

Special Categories of Witnesses

Certain categories raise recurring competency questions on the MBE: children, persons with mental impairment, judges, and jurors.

Children

Children may testify if they have sufficient intelligence and can understand the obligation to tell the truth. There is no fixed minimum age for competency under the Federal Rules.

Courts generally look at:

Whether the child can differentiate truth from lies
Whether the child understands that they must tell the truth in court
Whether the child can perceive, remember, and communicate basic facts about the events

Typical inquiry:

“If I say your shirt is green and it’s really red, is that the truth or a lie?”
“What happens if you tell a lie in court?”
“Do you promise to tell the truth today?”

If the child lacks these abilities, the judge may find the child incompetent as a witness. But age alone is not disqualifying; even very young children may be found competent if they pass the judge’s inquiry.

On the exam:

A child who can observe, recall, and communicate, and who understands the duty to tell the truth, is competent.
A child who cannot distinguish truth from lies, or cannot recall the events in any meaningful way, may be found incompetent.

Any remaining concerns about the child’s reliability are handled through cross-examination, not exclusion.

Mentally Impaired Persons

A person with mental impairment may testify if they can observe, recall, and communicate about the matter and understand the duty to tell the truth. The key is functional capacity, not diagnosis.

Questions to consider:

Can the witness perceive relevant events?
Can they remember those events well enough to give meaningful testimony?
Do they understand that they must tell the truth in court?

Mental illness (such as schizophrenia), intellectual disability, or dementia does not automatically disqualify a witness. Those conditions are usually grounds for impeachment and may reduce the weight of the testimony, but they rarely lead to a finding of absolute incompetence.

Only in extreme situations—such as a witness whose mental condition prevents any coherent communication—will the court rule the witness incompetent.

Judges and Jurors

Judges presiding over a case and jurors serving in that case are the main modern exceptions to the general presumption of competency.

Presiding judge (FRE 605): The judge may not testify as a witness in the trial over which they are presiding. No objection is required to preserve this issue for appeal.
Jurors (FRE 606(a)): Jurors may not testify as witnesses before the jury on which they serve. If a juror is called, the court must give the opposing party an opportunity to object outside the jury’s presence.

After a verdict, jurors are also restricted in what they can testify about regarding deliberations (FRE 606(b)).

Jurors generally may not testify about:

Statements made or incidents occurring during deliberations
The effect of anything on a juror’s mental processes in deciding the verdict
The reasoning or methods by which the jury reached its verdict (e.g., misunderstanding instructions, being swayed by sympathy)

Jurors may testify about:

Whether extraneous prejudicial information was improperly brought to the jury’s attention (e.g., unauthorized internet research, outside documents, news articles)
Whether any outside influence was improperly brought to bear on any juror (e.g., bribes, threats, improper communications from a bailiff or party)
Whether there was an error in entering the verdict on the form (clerical error)
In rare circumstances, whether the verdict was clearly tainted by overt racial bias: under the Supreme Court’s Peña-Rodriguez decision, if a juror makes a clear statement that racial animus was a significant motivating factor in the verdict, juror testimony may be allowed to impeach the verdict.

These rules protect the secrecy and integrity of deliberations, while allowing the court to address outside interference and serious misconduct.

Dead Man’s Statutes and Interested Witnesses

Key Term: Dead Man's Statute
A state law that may bar an interested party from testifying about communications or transactions with a deceased person in certain civil cases, typically when the testimony would be adverse to the decedent’s estate.

Dead Man’s Statutes:

Are not part of the Federal Rules of Evidence; there is no federal Dead Man’s Rule.
Exist in some states and vary in scope, but a common pattern is:
- They apply in civil cases.
- They bar an “interested person” (often a party or someone with a direct financial stake) from testifying to oral communications or transactions with the decedent.
- They apply when the testimony would be adverse to the decedent’s estate or successors.

Typical rationale: once the decedent is gone and cannot rebut the testimony, the law restricts interested parties from testifying about private conversations or dealings with the decedent.

Common exceptions in state statutes (these details may or may not appear on the MBE, but they explain the logic):

If the estate opens the door by offering evidence of the transaction or communication, the bar may be waived.
If the decedent’s own deposition or statement is admitted, the adverse party may be allowed to respond.
Some statutes allow the court to waive the bar in the interest of justice.

Dead Man’s Statutes are a competency rule: an otherwise competent witness is rendered incompetent to testify about certain matters because of their interest and the decedent’s unavailability.

State Law in Diversity Cases (FRE 601)

Under FRE 601, in federal civil actions and proceedings, witness competency is generally governed by the Federal Rules—except in civil cases where state law supplies the rule of decision (i.e., diversity cases and supplemental state-law claims). In those cases:

State law on competency applies to witnesses regarding that state-law claim or defense.
This includes any applicable state Dead Man’s Statute.

This means:

In a federal question case, federal competency rules control across the board.
In a civil diversity case (or a supplemental state-law claim in federal court), if state law would render the witness incompetent—for example, under a Dead Man’s Statute—the federal court must apply that rule when deciding the state-law claim.

This can be outcome-determinative: a party who could testify in a pure federal question case may be barred from testifying in a diversity case because of a state Dead Man’s Statute.

On the exam, when you see:

Federal court
Diversity jurisdiction
Testimony by a party about a conversation with a deceased adversary

you should immediately consider whether a state Dead Man’s Statute applies via FRE 601.

Expert Witnesses

So far we have focused on basic competency—who can take the stand at all. Expert witnesses must satisfy those same baseline requirements and also meet additional requirements for expert status.

Key Term: Expert Witness
A person qualified by specialized knowledge, skill, experience, training, or education who may give opinion testimony to help the fact-finder understand the evidence or determine a fact in issue.

Under FRE 702, an expert witness may testify if the court finds that:

The expert is qualified by knowledge, skill, experience, training, or education.
The expert’s testimony will help the trier of fact understand the evidence or determine a fact in issue (helpfulness requirement).
The testimony is based on sufficient facts or data.
The testimony is the product of reliable principles and methods.
The expert has reliably applied those principles and methods to the facts of the case.

This is often summarized as the Daubert reliability test: the judge acts as a gatekeeper to ensure expert testimony is both relevant and reliable.

Although the Rules do not list exclusive factors, courts applying Daubert commonly consider:

Whether the theory or technique can be and has been tested
Whether it has been subjected to peer review and publication
The known or potential error rate and standards controlling the technique’s operation
Whether it has gained general acceptance in the relevant scientific or technical community

Important contrasts with lay witnesses:

Experts may give opinions, including on ultimate issues (e.g., “the design was unreasonably dangerous”), except that in criminal cases they generally may not opine on whether the defendant had the mental state (intent, knowledge, etc.) that is an element of the crime or defense.
Experts do not need personal knowledge of the case facts. They can base opinions on:
- Facts they personally observed
- Facts presented at trial in the form of testimony or exhibits (often through hypotheticals)
- Facts or data that are not themselves admissible (e.g., hearsay), if experts in the field would reasonably rely on such information (FRE 703).
The expert may disclose the supporting inadmissible facts to the jury only if their probative value in helping the jury evaluate the opinion substantially outweighs their prejudicial effect.

The admissibility of expert testimony is therefore a qualification plus reliability question, not just a general competency question. The expert must be competent as a witness and must also clear the FRE 702 reliability screen.

On appeal, a trial court’s decision to admit or exclude expert testimony is reviewed for abuse of discretion, reflecting the judge’s broad gatekeeping role.

Lay Witness Opinion vs. Competency

Although lay witnesses usually testify to facts, the Federal Rules allow limited opinion testimony from lay witnesses under FRE 701 if:

It is rationally based on the witness’s perception;
It is helpful to understanding the witness’s testimony or determining a fact in issue; and
It is not based on scientific, technical, or specialized knowledge (that domain is reserved for experts).

Examples of permissible lay opinions:

Estimates of speed (“the car was going very fast, maybe 70 mph in a 30 mph zone”)
Apparent intoxication (“he seemed drunk; he was slurring his words and stumbling”)
Emotional state (“she looked terrified”)
General physical condition or age

These rules govern what a competent witness may say; they do not typically affect whether the witness is competent to testify in the first place.

Exclusion (Sequestration) of Witnesses vs. Competency

Under FRE 615, at the request of a party or on its own, the court must order witnesses (other than parties and certain essential persons) excluded from the courtroom so they cannot hear the testimony of others. This is often called “sequestration.” It is designed to reduce the risk of witnesses tailoring their testimony.

Sequestration:

Does not make a witness incompetent.
Affects when and how the witness may be present in the courtroom, not whether the witness may testify.
Has specific exceptions (for example, a party to the case generally cannot be excluded).

On the MBE, do not confuse sequestration (a mode-of-presentation issue) with competency (a threshold qualification issue).

Worked Example 1.1

A 7-year-old child witnessed a car accident and is called to testify. The child can describe what happened and promises to tell the truth. The opposing party objects, arguing the child is too young.

Answer:
The child may testify if the court finds the child can observe, recall, and communicate about the event and understands the duty to tell the truth. There is no fixed age bar under the Federal Rules. The judge will conduct a brief inquiry, but if the child passes, the child is competent and any concerns go to weight, not admissibility.

Worked Example 1.2

In a federal diversity case, a party seeks to testify about a conversation with the now-deceased opposing party. The state where the court sits has a Dead Man’s Statute.

Answer:
The federal court will apply the state’s Dead Man’s Statute to determine if the witness is competent to testify about the conversation, because in a civil case where state law supplies the rule of decision, state competency rules, including any Dead Man’s Statute, govern under FRE 601. If the statute applies, the witness may be incompetent to testify about that transaction.

Worked Example 1.3

A party offers an engineer as an expert witness on bridge safety. The engineer has a bachelor’s degree in civil engineering, 20 years’ experience designing bridges, and has published on bridge failures. The engineer’s methods are ones commonly used and tested in the field. The court asks about the engineer’s qualifications and the methods used, then allows the testimony.

Answer:
The court must find the engineer is qualified and that the testimony is based on sufficient facts or data and reliable principles and methods reliably applied to the facts. The judge acts as a Daubert gatekeeper for expert evidence, in addition to ensuring the engineer meets basic competency requirements (personal knowledge of any observations, oath, ability to communicate). The engineer’s practical experience can satisfy the qualification requirement; there is no need for a PhD.

Worked Example 1.4

In a civil trial, after a verdict for the plaintiff, the losing defendant offers a juror’s affidavit stating that most jurors misunderstood the judge’s instructions and based their decision on sympathy for the plaintiff’s injured child.

Answer:
The affidavit is inadmissible. Under FRE 606(b), a juror may not testify about statements made or incidents that occurred during deliberations or about the effect of anything on any juror’s mental processes. Misunderstanding the law and being influenced by sympathy are internal deliberation issues, not extraneous information or outside influence, so the juror is incompetent to testify to them for purposes of impeaching the verdict.

Worked Example 1.5

A key eyewitness speaks only Spanish. The court appoints a certified interpreter who takes an oath to translate truthfully. The opposing party objects that the witness cannot testify because the jury will not understand Spanish.

Answer:
The objection should be overruled. The witness can communicate through a qualified interpreter who has taken an oath to translate truthfully. Under FRE 604, interpreters are expressly authorized and, once appointed and sworn, the witness’s ability to communicate through them satisfies the competency requirement. The interpreter can be cross-examined about accuracy and bias.

Worked Example 1.6

In a federal criminal trial, the prosecution offers a psychiatrist to testify that, in the expert’s opinion, the defendant “knew what he was doing and intended to kill the victim.” Defense counsel objects.

Answer:
The objection should be sustained in part. The psychiatrist may testify about the defendant’s mental condition, diagnoses, and how such conditions typically affect perception and behavior, and may answer hypothetical questions. However, under FRE 704(b), in a criminal case an expert may not state an opinion about whether the defendant did or did not have the mental state or condition constituting an element of the crime. The expert cannot directly opine that the defendant “intended to kill.”

Exam Warning

The MBE may test whether a witness’s lack of personal knowledge, refusal to take an oath, or inability to understand the duty to tell the truth makes them incompetent. It may also test the special rules for judges, jurors, and Dead Man’s Statutes. Always check:

Does the witness have personal knowledge (if a lay witness)?
Has the witness taken an oath or affirmation and do they appreciate the duty to tell the truth?
Can the witness communicate, with or without an interpreter?
Is there any specific rule (judge, juror, state Dead Man’s Statute) that disqualifies this witness?
If an expert, is the witness properly qualified and is the methodology reliable under FRE 702?

Only after those questions should you move on to hearsay, privileges, or substantive issues.

Revision Tip

For MBE questions on witness qualifications, remember:

Competency is broadly presumed; incompetency is rare and usually limited to failure of personal knowledge (for lay witnesses), oath/affirmation, or ability to communicate.
Age, mental illness, bias, interest, and prior convictions usually affect credibility, not competency.
In diversity cases, state competency rules—including Dead Man’s Statutes—can override the general federal presumption via FRE 601.
For experts, think: qualification + reliability + helpfulness, with the judge as gatekeeper under FRE 702 and Daubert.
Jurors and the presiding judge are uniquely restricted as witnesses; juror testimony about deliberations is tightly limited.

Key Point Checklist

This article has covered the following key knowledge points:

Under the Federal Rules, all persons are presumed competent to testify unless a rule (such as FRE 605 or 606, or an applicable state statute via FRE 601) provides otherwise.
Every witness must have personal knowledge (for lay testimony), be willing to take an oath or affirmation, and be able to communicate (with an interpreter if necessary).
Children and mentally impaired persons may testify if they understand the duty to tell the truth and can observe, recall, and relate relevant facts; weaknesses ordinarily go to weight, not admissibility.
Presiding judges and jurors in a case are uniquely restricted: judges may not testify at all, and jurors are barred from testifying about deliberations except for narrow exceptions (extraneous information, outside influence, clerical errors, and certain clear statements of racial bias).
Dead Man’s Statutes are state competency rules that may bar interested witnesses from testifying about transactions or communications with a decedent; they apply in federal diversity cases when state law supplies the rule of decision.
Expert witnesses must meet both basic competency requirements and the additional FRE 702 standards of qualification, reliability, and helpfulness; the court acts as a Daubert gatekeeper and may exclude unreliable expert opinions.
Lay witnesses may give limited opinion testimony based on their perceptions if it is helpful and not based on specialized knowledge; more technical opinions require a qualified expert.
Sequestration of witnesses under FRE 615 affects the order and setting of testimony but does not itself make a witness incompetent.

Key Terms and Concepts

Competency
Personal Knowledge
Oath or Affirmation
Interpreter
Dead Man's Statute
Expert Witness

Relevancy and reasons for excluding relevant evidence - Qual...