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How to Write a Hypothesis for a Psychology Lab Report
A hypothesis is a testable prediction about how two or more variables are related. In a psychology lab report it sits at the end of the Introduction, and everything after it, from the design to the statistics to the discussion, is built to test it.
A strong hypothesis names the variables, states the direction when you have a reason to, and can be shown wrong by your data. This guide covers the three versions you may be asked for, how to choose between them, and what weak and strong wording looks like. If you are still sorting out your variables, start with our independent and dependent variables guide.
The three versions of a hypothesis
The examples below all use one study: does sleep restriction affect reaction time?
| Version | What it says | Example | Symbols |
|---|---|---|---|
| Directional (research) hypothesis | Predicts which way the difference goes | Participants who sleep 4 hours will have slower mean reaction times on a simple reaction task than participants who sleep 8 hours. | H1: mean RT (4 h) > mean RT (8 h) |
| Non-directional (research) hypothesis | Predicts a difference without a direction | Mean reaction time will differ between participants who sleep 4 hours and participants who sleep 8 hours. | H1: mean RT (4 h) is not equal to mean RT (8 h) |
| Null hypothesis | Predicts no difference or no relationship | There will be no difference in mean reaction time between participants who sleep 4 hours and participants who sleep 8 hours. | H0: mean RT (4 h) = mean RT (8 h) |
Directional or non-directional?
Use a directional hypothesis when earlier research gives you a reason to predict the direction, and cite that research in your introduction. Use a non-directional hypothesis when the evidence is mixed or the area is new.
The choice matters for your analysis. A directional hypothesis is usually tested with a one-tailed test and a non-directional one with a two-tailed test. Many instructors expect two-tailed tests by default, so check your rubric before you choose.
Most courses want the research hypothesis in your report. The null hypothesis is what the statistical test actually evaluates, so some courses ask for both. If yours does, write the research hypothesis first and the null second.
Weak vs strong hypotheses
| Weak | Why it loses marks | Strong |
|---|---|---|
| "Sleep affects performance." | No measurable variables, no direction | "Participants who sleep 4 hours will make more errors on a 10-minute attention task than participants who sleep 8 hours." |
| "Music helps studying." | Vague, and "helps" is not measured | "Students who study a passage with instrumental music playing will recall fewer facts on a 20-item quiz than students who study in silence." (Use the direction your own literature supports.) |
| "Social media is bad for mental health." | Too broad to test, and implies cause | "Higher average daily phone screen time will be positively associated with PHQ-9 scores in university students." |
| "People remember pictures better." | No comparison group, no measure | "Participants shown 20 words paired with images will recall more words after a 10-minute delay than participants shown the 20 words alone." |
| "This study will prove that retrieval practice works." | "Prove" overclaims, and the variables are undefined | "Participants who spend ten minutes on retrieval practice will score higher on a delayed test than participants who spend ten minutes re-reading." |
| "People are happier when life has meaning." | Variables are not operationalized | "Scores on the Satisfaction With Life Scale will be positively correlated with scores on the Meaning in Life Questionnaire." |
To define variables the way the strong versions do, see our guide to operational definition examples in psychology.
Hypotheses for correlational and quasi-experimental studies
If you did not manipulate a variable, do not write a causal hypothesis.
- Correlational: "Higher scores on X will be associated with higher scores on Y." Say positive or negative if you can justify it.
- Quasi-experimental, for example comparing first-year and fourth-year students: "Fourth-year students will report lower exam anxiety than first-year students." The groups already existed, so avoid words like "causes" or "leads to".
Where the hypothesis goes in your report
The hypothesis comes at the end of the Introduction, after you have reviewed the research and stated the gap or question. A common pattern is one sentence of purpose followed by the hypothesis: "The present study examined whether ... It was hypothesized that ..."
If your study has more than one hypothesis, number them or give each its own sentence, in the same order as your analyses in the Results.
Hypothesis, prediction, research question and aim
| Term | What it is | Example |
|---|---|---|
| Research question | An open question | Does sleep affect reaction time? |
| Aim | What the study is trying to do | To examine the effect of sleep restriction on reaction time |
| Hypothesis | A specific, testable prediction | Participants who sleep 4 hours will have slower mean reaction times than those who sleep 8 hours |
Some courses use "prediction" and "hypothesis" interchangeably. If yours separates them, the hypothesis is the general statement and the prediction is the specific outcome for your measures.
Mistakes to avoid
- Writing a hypothesis you cannot test with your design or data.
- Using "prove". A study can support a hypothesis, not prove it.
- Leaving out a variable, or leaving variables undefined.
- Changing the hypothesis after you see the results. This practice is called HARKing, hypothesizing after the results are known, and it is treated as poor research practice (Kerr, 1998). If the result is the opposite of what you predicted, keep the hypothesis and explain the result in your Discussion.
- Writing a question instead of a statement.
Frequently asked questions
Can I have more than one hypothesis?
Yes, if your design has more than one comparison or relationship. Write one clear hypothesis for each, in the order you will analyse them.
Do I have to write the null hypothesis?
It depends on the course. Many reports state only the research hypothesis, and the null is implied by the statistical test. If your instructor asks for both, state them as a pair.
Can a hypothesis be a question?
No. A question is a research question. A hypothesis is a statement that your data can support or fail to support.
What if my hypothesis is not supported?
That is a valid result. Report it as "not supported" and explain it in the Discussion. Our guide to the discussion section shows how.
Is a hypothesis the same as a theory?
No. A theory is a broad, well-supported explanation of a phenomenon. A hypothesis is a specific prediction, often derived from a theory, that a study tests.
Related guides and tools
- Operational definition examples in psychology
- Independent and dependent variables: 25 psychology examples
- How to write an APA 7 psychology lab report
- How to write the discussion section of a psychology lab report
- Psychology assignment help
Sources: APA Dictionary of Psychology, entries "hypothesis" and "null hypothesis" (dictionary.apa.org). Kerr, N. L. (1998). HARKing: Hypothesizing after the results are known. Personality and Social Psychology Review, 2(3), 196-217. Steger, M. F., Frazier, P., Oishi, S., & Kaler, M. (2006). The Meaning in Life Questionnaire. Journal of Counseling Psychology, 53(1), 80-93. Diener, E., Emmons, R. A., Larsen, R. J., & Griffin, S. (1985). The Satisfaction With Life Scale. Journal of Personality Assessment, 49(1), 71-75.
Examples in this guide are illustrative. Check your course instructions and rubric before you submit.
