PastPaperHero | Presentation of evidence

Learning Outcomes

This article explains the competency of witnesses under the Federal Rules of Evidence and related state law, including:

The general presumption that all persons are competent to testify and the limited circumstances in which a witness is actually disqualified
How personal knowledge, the ability to communicate, and the oath or affirmation requirement operate as baseline competency gates
How courts evaluate children, mentally impaired witnesses, and convicted felons, and distinguish functional competency from mere credibility concerns
Why judges and sitting jurors are uniquely disqualified from testifying and how the rules governing juror testimony during and after trial work
When and how state Dead Man’s Statutes can bar an interested witness from testifying in civil actions, particularly in diversity cases
When state law, rather than the Federal Rules, governs competency in civil diversity cases under Rule 601
How competency is determined as a preliminary question by the judge under Rule 104(a), and what evidence the court may consider
How to separate genuine competency issues from attacks on credibility when selecting MBE answer choices
How competency objections must be raised, preserved, and reviewed on appeal, including harmless and plain error analysis
How interpreters and other accommodations affect, but do not usually defeat, a witness’s competency to testify

MBE Syllabus

For the MBE, you are required to understand who may testify as a witness in federal court and in diversity cases, with a focus on the following syllabus points:

General presumption of competence under the Federal Rules of Evidence
Requirements of personal knowledge, ability to communicate, and oath or affirmation
Treatment of children, mentally impaired persons, and convicted felons
Absolute bars on judge and juror testimony and the limited exceptions for juror testimony
Nature and operation of Dead Man’s Statutes as state-law competency rules
When state law governs competency in civil diversity cases (Rule 601)
Distinction between competency, relevance, and credibility
Judge’s role under FRE 104 in deciding preliminary questions of competency
Appellate review of competency rulings and the plain error rule

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 witness competency under the Federal Rules of Evidence?
1. Personal knowledge of the matter
2. Ability to communicate
3. Absence of criminal record
4. Oath or affirmation to testify truthfully
A 7-year-old child is called as a witness. What must the judge determine before allowing the child to testify?
1. That the child has never lied before
2. That the child understands the duty to tell the truth and can observe, remember, and communicate
3. That the child has a parent present in court
4. That the child has attended school
Under the Federal Rules, who is absolutely barred from testifying in a trial over which they preside?
1. A convicted felon
2. A juror
3. The presiding judge
4. A party to the case
In a federal diversity case, which law governs witness competency?
1. Federal Rules of Evidence
2. State law of the forum state
3. Law of the witness’s domicile
4. Law of the state where the contract was made

Introduction

Competency determines whether a person is legally permitted to give evidence as a witness in court. Under the Federal Rules of Evidence there is a broad presumption that people are competent to testify. Most attacks that sound like “this witness can’t testify” actually go to credibility and weight, not competency.

Key Term: Competency of Witnesses
The legal fitness of a person to give evidence as a witness in court, determined by the applicable rules of evidence.

Federal Rule of Evidence 601 provides that every person is competent to be a witness unless a specific rule provides otherwise. Old common-law disqualifications (for example, being a party to the case, having a financial interest, lacking religious belief, or having a prior felony conviction) have largely been abolished. The Rules instead impose a small set of baseline requirements for all witnesses and a few targeted disqualifications.

Competency is a preliminary question of law decided by the judge, not the jury. Under FRE 104(a), the court decides whether a witness meets the competency standard and may consider information that would not itself be admissible at trial (subject to privilege rules). The judge applies a preponderance-of-the-evidence standard: is it more likely than not that the witness satisfies the minimum requirements?

This division of roles is tested frequently:

The judge decides whether the witness can take the stand at all (competency and other admissibility issues).
The jury decides how much weight to give the testimony (credibility and persuasiveness).

Understanding where that line is drawn is important for MBE questions that ask whether the testimony should be admitted at all, or simply left to the jury to evaluate.

General Rule of Competency

The starting point for any MBE question is:

Everyone is competent to testify unless a rule says otherwise.
Challenges based on age, mental condition, bias, or criminal record almost always go to weight and credibility, not to competency.

This presumption favors admissibility. The jury then decides how much weight to give the testimony.

Under Rule 601:

No general disqualification for being a party or having a stake in the case
No disqualification for lack of religious belief or for holding a particular religion
No automatic bar based solely on felony conviction or mental illness

If an answer choice suggests “X cannot testify because she is a party,” it is almost certainly wrong under the Federal Rules.

Basic Requirements for All Witnesses

Despite the broad presumption, a witness must satisfy several baseline requirements.

Personal Knowledge

Federal Rule of Evidence 602 requires that a witness testify only from personal knowledge.

Key Term: Personal Knowledge
Direct knowledge of a fact gained through a witness’s own perception (e.g., seeing, hearing, smelling), shown by evidence sufficient for a reasonable juror to find that the witness perceived the matter.

Key points:

The witness does not have to prove personal knowledge with extrinsic evidence; the witness’s own testimony (“I saw…”, “I heard…”) usually suffices.
The judge must find evidence “sufficient to support a finding” of personal knowledge. This is a low threshold.
If the witness’s testimony makes clear that they are speculating or repeating what others said, the court should exclude that portion for lack of personal knowledge.
Lack of detail or poor memory usually weakens credibility, but does not automatically defeat personal knowledge. A witness can say, “It was sometime in the spring, I think in April,” and still have personal knowledge of the key events.

On MBE questions:

A police officer reading from an accident report about what another officer saw has no personal knowledge of those facts; the officer has personal knowledge only of what he himself perceived.
A bystander who heard the crash but did not see the impact nonetheless has personal knowledge of what he heard and what he saw immediately afterward.

Note: Expert witnesses are different; they may rely on facts and data they did not personally perceive. For lay witnesses, lack of personal perception is a competency problem.

Oath or Affirmation

Federal Rule of Evidence 603 requires an oath or affirmation to tell the truth.

Key Term: Oath or Affirmation
A formal promise to testify truthfully, in a form designed to impress the duty of truthfulness on the witness’s conscience. It can be religious or secular.

The oath requirement is very flexible:

A formal religious oath is not required. A secular promise to tell the truth is enough.
The exact wording does not matter; what matters is that the witness understands they must be truthful.
For children or persons with limited language skills, the court may phrase it in age-appropriate terms (“Do you promise to tell us what really happened?”).
A witness who refuses to state that they will testify truthfully is not competent. The court cannot force testimony from someone who refuses to accept the duty of truthfulness.

The oath requirement is one of the few true “on/off” gates to competency: if a witness refuses or cannot meaningfully affirm an obligation to tell the truth, testimony is barred.

Ability to Communicate

Competency also requires that the witness be able to observe, remember, and communicate the testimony in a way that the fact-finder can understand.

The witness must be able to communicate answers intelligibly, whether orally, in writing, through sign language, or through an interpreter.
Physical limitations (such as deafness or muteness) do not by themselves render a witness incompetent. They simply require accommodations.
Limited vocabulary, accent, or nervousness rarely make someone incompetent; these are credibility issues for the jury.

Key Term: Interpreter
A qualified person who translates a witness’s testimony into a language the court and jury can understand; the interpreter must take an oath to provide a true translation and is treated as a witness for competency and impeachment purposes.

If a witness speaks a different language or is hearing-impaired, the court appoints an interpreter (FRE 604). Important points:

The interpreter must be qualified and must swear an oath to translate truthfully.
The interpreter is subject to impeachment like any other witness (bias, prior inconsistent statements, etc.).
The witness is still the source of evidence. The interpreter is just a tool for communication, not an additional witness offering independent testimony.

On the MBE, if translation accuracy is challenged, that goes to the interpreter’s credibility, not to the actual witness’s competence (unless the communication problem is so severe that the fact-finder cannot understand the evidence at all).

Understanding the Duty to Tell the Truth

The witness must appreciate the obligation to testify truthfully. This is most often tested with children or mentally impaired witnesses. The judge may briefly question the witness (a voir dire) about:

The difference between truth and lies
Whether lying is wrong
Whether they promise to tell the truth in court

If the witness cannot demonstrate even a basic understanding of the duty of truthfulness, the judge may find the witness incompetent.

This is a functional test. A child need not articulate abstract moral philosophy; it is usually enough if the child can, in simple terms, distinguish truth from lies and understands there is a duty to be truthful.

Ability to Communicate and Understand Truthfulness

For many witnesses the key competency question is not age or diagnosis, but functional ability:

Can the person perceive relevant events?
Can they remember those events long enough to testify?
Can they communicate what they perceived in a way the court can understand?
Do they understand that they must tell the truth?

Impairments (such as memory gaps, cognitive limitations, or limited language skills) usually go to weight, not to admissibility, unless they are so extreme that meaningful testimony is impossible.

Examples:

A witness who cannot remember exact dates but remembers the sequence of events is competent; the fuzzy dates go to credibility.
A witness with dementia who can accurately recall events from the relevant period, but gets confused about unrelated matters, may still be competent if she can describe what she perceived and affirm a duty to tell the truth.
A heavily intoxicated person at the time of the events may still be competent to testify later; intoxication goes to reliability and weight rather than baseline competence.

On the MBE, answer choices that treat ordinary limitations as competency problems are often wrong. Unless the facts indicate that the witness literally cannot understand truth vs. lie or cannot communicate at all, the testimony should come in and the jury evaluates it.

Special Categories of Witnesses

Certain categories appear frequently on the MBE. Under modern rules, they are usually competent, but with special attention to the basic requirements.

Children

Children may testify in federal court if they can:

Observe events
Remember them
Communicate them
Understand the duty to tell the truth

There is no fixed minimum age in the Federal Rules.

Key Term: Child Witness
A witness who is a minor; competency is assessed individually based on the child’s ability to perceive, remember, communicate, and understand the duty of truthfulness, rather than by age alone.

The judge will typically:

Question the child in a brief, informal voir dire outside the jury’s hearing.
Ask simple questions illustrating truth and lies (“If I said my shirt is green when it is blue, is that the truth or a lie?”).
Ask whether the child will tell the truth in court and whether lying is bad or gets you in trouble.

If the child passes this threshold, the child is competent. Weaknesses (short attention span, limited detail, confusion about time) affect credibility and may justify the use of more leading questions, but they do not bar the testimony.

On the exam, beware of answer choices that impose a minimum age (“under 10 cannot testify”). That reflects outdated common-law rules, not the Federal Rules.

Mentally Incapacitated Persons

Mental illness, developmental disability, or cognitive impairment does not automatically disqualify a witness.

Key Term: Mentally Incapacitated Witness
A witness with mental illness, developmental disability, or cognitive impairment; competent to testify if able to perceive, remember, communicate, and understand the obligation to tell the truth.

Key exam points:

Diagnosis alone is irrelevant to competency. A label such as “schizophrenia” or “intellectual disability” does not answer whether the person can testify.
The judge focuses on whether this individual, at the time of testifying, meets the basic functional requirements.
Mental conditions usually go to weight and can be used for impeachment (cross-examination, expert testimony), not exclusion.

For example, a witness who experiences hallucinations may still be competent if she can clearly describe what she saw at the scene and understands that she must tell the truth. The hallucinations can be used by the opposing party to argue that the witness is unreliable, but they do not automatically bar testimony.

Convicted Felons and Interested Witnesses

Under modern rules, neither a criminal record nor a financial stake in the case makes a witness incompetent.

Key Term: Interested Witness
A witness who has a direct stake in the outcome of the case (for example, a party, an employee of a party, or a beneficiary of the estate); competent to testify, though interest may be used for impeachment.

Points to remember:

A convicted felon can testify like any other witness; prior convictions may be used to impeach credibility under FRE 609, but the witness remains competent.
Parties themselves are fully competent witnesses. There is no rule that a plaintiff or defendant cannot testify in their own case.
Financial interest or bias is a classic ground for cross-examination and argument, not for disqualification.

Old common-law rules that barred parties, interested witnesses, non-believers, or felons from testifying have been rejected in federal practice.

Special Disqualifications

Modern evidence law retains a few absolute disqualifications, chiefly related to the structure of the trial itself.

Judges and Jurors

Federal Rules 605 and 606 contain strict rules for judicial and juror testimony.

Key Term: Judge as Witness
The presiding judge in a case; absolutely barred from testifying as a witness in that trial for any party on any issue.

The judge may not testify in the trial over which they are presiding.
No objection is required; if it happens, it is reversible error. The rule is self-executing.

The ban is broad: the judge cannot testify about facts, character of a party, or anything else. If the judge has personal knowledge of disputed facts, the proper remedy is recusal, not testimony.

Key Term: Juror Testimony Rule
The rule limiting juror testimony about deliberations; jurors generally may not testify about statements or mental processes in the jury room, but may testify about extraneous prejudicial information, outside influences, or certain explicit statements of racial bias.

Two distinct rules apply to jurors:

Juror as witness at trial (FRE 606(a))
- A juror in the case cannot be called to testify before their fellow jurors.
- If a juror has personal knowledge of a fact relevant to the trial, the court must excuse that juror and may permit testimony only if the person is no longer serving as a juror in that case.
- The goal is to avoid the spectacle and unfairness of a juror both participating in deliberations and serving as a fact witness.
Juror testifying after verdict (FRE 606(b))

During a later proceeding (e.g., a motion for new trial), a juror may not testify about:
- What was said in deliberations
- The effect of anything on any juror’s mind or emotions
- The jurors’ mental processes in reaching the verdict
But a juror may testify about:
- Extraneous prejudicial information improperly brought to the jury’s attention (e.g., a newspaper article smuggled into the jury room, internet research, a dictionary definition used contrary to the court’s instructions)
- Outside influence on any juror (e.g., bribes, threats, contact from parties, pressure from court officers)
- A clear statement that racial stereotypes or animus motivated the verdict in a significant way

These rules protect the integrity and finality of jury deliberations while permitting inquiry into improper outside influence and egregious racial bias.

Exam Warning

In federal court, the presiding judge and sitting jurors are not competent witnesses in the very trial over which they preside or sit. Calling the judge or a juror as a witness in that case is improper and the testimony will be excluded.

On the MBE:

If an answer choice suggests “the juror can testify to what was said in deliberations about how the law should be applied,” it is wrong.
If an answer choice suggests “the juror can testify that a bailiff threatened him during deliberations,” that fits the outside-influence exception and may be admitted.

Dead Man’s Statutes

Dead Man’s Statutes are state competency rules; the Federal Rules contain no such statute.

Key Term: Dead Man’s Statute
A state law that, in certain civil cases, bars an interested party or witness from testifying about communications or transactions with a deceased person when the testimony is offered against the decedent’s estate or successors.

Typical elements (specific statutes vary by state):

Applies only in civil cases.
Applies when testimony is offered against the decedent’s estate or those claiming under the decedent (heirs, devisees).
Bars testimony by an interested witness (often a party or person with a financial stake in the case).
Bars testimony about communications or transactions with the decedent (e.g., “He told me I could have the farm,” “We made a handshake agreement that he would forgive the loan”).

Common variations include:

Requiring corroboration of the interested witness’s testimony.
Allowing the estate’s representative to “open the door” by testifying about the transaction, which then permits the adverse party to testify in response.
Allowing the protected party to waive the protection by failing to object or by calling the interested witness themselves.

MBE implications:

Federal Rules of Evidence do not contain a Dead Man’s Statute, so in a pure federal question case there is no such bar.
In diversity cases, Rule 601 may incorporate state Dead Man’s rules (discussed below).
If the fact pattern identifies the forum state as having a Dead Man’s Statute, you must apply it in a civil case where state law supplies the rule of decision.

State Law in Diversity Cases

In federal diversity cases, Federal Rule of Evidence 601 gives state law an important role in competency.

Key Term: State Law Competency Rules
State-law provisions that govern whether a witness is competent in civil actions where state law supplies the rule of decision (for example, Dead Man’s Statutes or specialized state disqualifications).

Rule 601 provides:

In civil actions and proceedings, when state law supplies the rule of decision for a claim or defense (as in diversity or supplemental state-law claims), state law governs witness competency.
This includes state Dead Man’s Statutes and any other state competency rules (such as statutes making certain felons incompetent as witnesses in civil cases).
In federal question cases, or for issues governed by federal law, federal competency rules (the FRE) apply.

Thus, on the MBE:

If the case is a federal diversity civil case and the question involves an interested witness testifying about a conversation with a decedent, you must ask whether the forum state has a Dead Man’s Statute and apply it.
If the case is a § 1983 civil rights action or other federal question case, the Federal Rules control and Dead Man’s Statutes do not apply, even if the forum state has one.
In a mixed case (both federal and state claims), the court may apply federal competency rules to the federal claims and state rules to the state claims, though in practice the standard is usually harmonized at the federal level unless the state rule is outcome-determinative.

Revision Tip

In MBE diversity questions, always check for state-specific competency rules—especially a Dead Man’s Statute—because Rule 601 requires federal courts to follow those state rules for state-law claims.

Competency vs. Credibility

Many MBE distractors mislabel credibility problems as competency problems. Keep these distinctions clear:

Competency focuses on threshold ability: personal knowledge, ability to communicate, and understanding of the duty to tell the truth. It is a gatekeeping question for the judge under Rule 104(a).
Credibility involves bias, interest, prior convictions, inconsistent statements, poor memory, intoxication at the time of observation, or mental illness affecting reliability. It is for the jury to weigh, using cross-examination and impeachment rules.

Examples of competency issues:

Witness cannot communicate at all in any form that the court can understand.
Child cannot distinguish a truth from a lie even in simple examples.
Witness refuses to promise to tell the truth.

Examples of credibility-only issues:

Witness is the plaintiff’s spouse and strongly biased in plaintiff’s favor.
Witness was drunk at the time of the accident.
Witness has a prior conviction for fraud.
Witness has an inconsistent prior statement.

On the MBE:

If a question asks whether the witness is “competent” and the only problem is bias, diagnosis, or criminal record, the correct answer is almost always that the witness is competent and those issues go to credibility.
If the question describes a child who understands truth vs. lies and can describe events, any answer that says “child is incompetent due to age” is wrong.

Who Decides Competency and How

Competency is a preliminary question for the judge (FRE 104(a)):

The judge decides whether the witness satisfies the competency requirements.
In making this decision, the judge is not bound by the rules of evidence (except those on privilege). The court can consider hearsay, affidavits, and other materials not admissible before the jury.
The standard is usually a preponderance of the evidence.

In practice:

The judge may examine the witness outside the jury’s presence (especially with children or mentally impaired witnesses) to avoid prejudice and confusion.
The judge may also consider testimony from parents, doctors, or others about the witness’s ability to perceive and communicate.
Once the judge finds the witness competent, challenges to the witness’s perceptions, memory, or bias are for the jury.

If the judge erroneously admits or excludes a witness on competency grounds, the ruling is reviewed on appeal under an abuse of discretion standard. The error must affect a substantial right to justify reversal; otherwise the harmless error rule applies.

Procedural Aspects and Preservation for Appeal

Competency rulings are evidentiary rulings. To challenge them on appeal:

A party opposing the witness must make a timely objection, stating the specific grounds (for example, “lack of personal knowledge,” “cannot understand the duty to tell the truth”).
If the judge excludes a witness the party wishes to call, counsel should make an offer of proof, describing on the record what the witness would say and why the witness is competent.
If no objection or offer of proof is made, appellate review is limited to plain error—an obvious error that affects substantial rights and seriously affects the fairness or integrity of the proceedings.

For the MBE, you may see a fact pattern where no one objected to a child’s testimony at trial, then a party tries to complain on appeal. Unless the incompetency is extreme and obvious, the appellate court will likely treat any error as waived.

Interpreters and Accommodated Testimony

Witnesses with language or physical limitations are generally competent if accommodations allow them to communicate and understand the oath.

Important points for the exam:

A deaf witness who testifies through a sign-language interpreter is competent if the basic requirements are met.
A witness who writes answers on paper can be competent if this is an effective way to communicate.
The interpreter must be neutral and qualified; parties can object if the interpreter is biased (for example, a close relative of a party).
Errors in translation usually go to credibility and weight; only when communication is so garbled that the jury cannot reliably understand the testimony would competency be implicated.

Worked Examples

Worked Example 1.1

A 6-year-old child is the only eyewitness to a car accident. The child is called to testify in federal court. The judge questions the child and finds she can describe what happened, remembers details, and promises to tell the truth. Is the child competent to testify?

Answer:
Yes. The child meets the requirements of personal knowledge, ability to communicate, and understanding the duty to tell the truth. There is no minimum age for competency under the Federal Rules.

Worked Example 1.2

In a federal diversity case, the plaintiff sues the estate of a deceased person. The forum state has a Dead Man’s Statute. The plaintiff, an interested party, seeks to testify about a conversation with the decedent. Is the plaintiff competent to testify?

Answer:
No. In a diversity case, state law governs competency for the state-law claim. The Dead Man’s Statute operates as a competency bar and prevents the interested plaintiff from testifying about communications with the decedent when the testimony is offered against the estate.

Worked Example 1.3

In a federal criminal trial, the defense calls a witness who has been diagnosed with schizophrenia. The witness says she saw the real perpetrator commit the crime. The prosecution moves to exclude her as incompetent solely based on her diagnosis. The witness is able to describe what she saw and promises to tell the truth. Should the court exclude her as incompetent?

Answer:
No. Mental illness by itself does not render a witness incompetent. The witness has personal knowledge, can communicate, and understands the duty of truthfulness. Her condition may be explored on cross-examination and may affect the weight of her testimony, but not admissibility.

Worked Example 1.4

In a federal civil case under diversity jurisdiction, a state statute provides that any convicted felon is incompetent as a witness in civil actions. The plaintiff (a convicted felon) wishes to testify in her negligence suit. Does the federal court apply the state statute?

Answer:
Yes. Under FRE 601, state competency rules apply to civil claims governed by state law in diversity cases. The state statute disqualifying felons would control, and the plaintiff would be incompetent to testify on that state-law negligence claim in that federal diversity action.

Worked Example 1.5

After a jury returns a verdict for the defendant in a civil case, one juror tells the losing plaintiff that during deliberations another juror said, “We can’t believe the plaintiff because people of her race are always lying.” The plaintiff moves for a new trial and wants that juror to testify about the racist statement. Is the juror competent to testify about it?

Answer:
Yes. Although jurors generally cannot testify about deliberations, an exception allows juror testimony when a juror makes a clear statement that race-based stereotypes or animus were a significant motivating factor in the verdict. The juror may testify about that statement.

Worked Example 1.6

During a federal criminal trial, a party seeks to call one of the jurors as a witness to testify to facts he observed before being empaneled—facts that are relevant to the defendant’s guilt. The juror would continue to serve on the jury after testifying. Is the juror competent to testify?

Answer:
No. A juror may not testify as a witness before the other jurors in the trial in which they are sitting. The court must not permit the juror to testify in that capacity; if the testimony is essential, the juror would need to be excused as a juror and then could testify as an ordinary witness.

Worked Example 1.7

In a federal negligence action, a bystander is called to testify that “My neighbor told me the defendant ran the red light.” The neighbor is not called as a witness. The defendant objects that the bystander lacks personal knowledge. Should the bystander’s testimony be excluded for lack of competency?

Answer:
Yes, to that statement. The bystander has no personal knowledge of whether the defendant ran the red light; he only knows what the neighbor said. Under Rule 602, he is not competent to testify about the content of the neighbor’s perceptions. The bystander may testify to what he himself saw or heard (including the fact that the neighbor made a statement), but not to adopt the neighbor’s account as his own observations.

Worked Example 1.8

In a federal diversity case, the forum state has no Dead Man’s Statute. The defendant estate argues that federal courts must adopt a common-law rule barring interested parties from testifying against decedents. Is that argument correct?

Answer:
No. There is no federal Dead Man’s rule. Under FRE 601, competency is governed by state law in civil cases where state law supplies the rule of decision. If the forum state has no Dead Man’s Statute, then there is no competency bar, and an interested party may testify against the estate, subject to ordinary impeachment.

Worked Example 1.9

At a federal criminal trial, the government calls a 10-year-old child. Defense counsel objects, arguing that “children under 12 are incompetent in this state.” The judge finds the child understands the duty to tell the truth and can describe the events. Should the objection be sustained?

Answer:
No. In a federal criminal case, federal competency rules apply. There is no categorical age cutoff under the Federal Rules. Because the child can perceive, remember, communicate, and understands the duty of truthfulness, the child is competent even if state law would impose an age-based bar.

Key Point Checklist

This article has covered the following key knowledge points:

The Federal Rules presume all persons competent to testify unless a specific rule provides otherwise (FRE 601).
Basic competency requires personal knowledge, the ability to communicate, and a meaningful oath or affirmation to testify truthfully.
Children and mentally impaired persons are competent if they can observe, remember, communicate, and understand the obligation to tell the truth; age or diagnosis alone is not disqualifying.
Criminal convictions, financial interest in the outcome, lack of religious belief, or party status do not render a witness incompetent; they go to credibility and impeachment.
The presiding judge may not testify in the case they are trying, and sitting jurors are restricted from testifying both at trial and about deliberations, with narrow exceptions for extraneous influence and explicit racial bias.
Dead Man’s Statutes are state-law competency rules that may bar interested witnesses from testifying about communications with a decedent in certain civil cases.
In civil diversity cases (and state-law claims in federal court), state law governs competency when state law supplies the rule of decision, including Dead Man’s Statutes and other state competency rules.
Competency is decided by the judge as a preliminary question under FRE 104(a); most witness weaknesses are credibility issues for the jury, not grounds for incompetency.
Interpreters must be qualified, take an oath, and are treated as witnesses for impeachment, but do not create a separate competency hurdle for the actual witness.
Competency objections and rulings are evidentiary issues; they must be preserved by timely objection or offer of proof, and are reviewed on appeal for abuse of discretion, subject to the harmless and plain error rules.

Key Terms and Concepts

Competency of Witnesses
Personal Knowledge
Oath or Affirmation
Interpreter
Child Witness
Mentally Incapacitated Witness
Interested Witness
Judge as Witness
Juror Testimony Rule
Dead Man’s Statute
State Law Competency Rules

Presentation of evidence - Competency of witnesses