Plural of “hypothesis”

The plural of hypothesis is hypotheses.

Why

"Hypothesis" comes from Greek "hypothesis," literally "a placing under" (a foundation or supposition), built from "hypo-" (under) and the same "thesis" root discussed on its own page, and it follows the identical Greek -is/-es plural pattern: the plural adds a full extra syllable, "hy-POTH-uh-seez."

Because "hypothesis" is a core term in the scientific method taught from early education onward, most English speakers encounter both its singular and plural forms constantly through schooling, which likely explains why the word shows almost no drift toward a regularized "hypothesises" even among non-specialist writers, unlike some rarer members of the same -is/-es family. The distinction between a "hypothesis" and a scientific "law" is also worth noting alongside the theory distinction: a law describes a consistent, observed pattern (like the law of gravity) without necessarily explaining why it happens, while a hypothesis and eventually a theory attempt the explanation itself — three separate scientific terms often confused by the general public despite having quite distinct meanings in careful scientific usage.

Usage note

In formal scientific writing, a careful distinction is often maintained between a "hypothesis" (a specific, testable proposed explanation) and a broader "theory" (a well-substantiated explanation supported by extensive evidence) — a distinction unrelated to the word's plural, but one that trips up general readers almost as often as the crisis/theses-style plural pattern itself does.

The adjective "hypothetical," derived from "hypothesis," is fully regular and carries no trace of the noun's irregular plural, and it has drifted into much broader everyday use ("a hypothetical situation") than the noun itself, which stays largely confined to formal scientific and argumentative writing. Grant-writing and research-proposal guides frequently instruct scientists to state their hypotheses explicitly and separately at the start of a proposal, a formal convention that reinforces the word's plural in professional scientific writing far more consistently than most other technical vocabulary receives. Introductory statistics courses spend considerable time specifically on the null hypothesis, a formal baseline assumption of no effect that students must learn to state and test correctly before their own hypotheses can be properly evaluated.

Common real mistakes

The pronunciation shift between singular and plural causes more real confusion than the spelling does, since many speakers, especially in casual conversation, simply keep the singular's vowel sound when saying "hypotheses" aloud; the written overregularization "hypothesises" does occur but is rarer than the equivalent error with more everyday words in the same family. A further common misunderstanding, distinct from the grammar itself: casual speech sometimes uses "theory" where "hypothesis" would be the more precise term ("I have a theory about why this happened"), a meaning slip that doesn't affect the plural pattern but does affect how precisely the underlying idea is being communicated.

The wider pattern

"Hypothesis/hypotheses" is one of the largest and most frequently used members of the Greek -is/-es family that also includes "thesis/theses," "crisis/crises," "analysis/analyses," and "parenthesis/parentheses," all sharing the identical, still fully productive plural pattern.

Examples

• The researchers proposed three competing hypotheses to explain the data.
• Her hypothesis predicted a measurable change in temperature.
• Both hypotheses were tested using the same experimental method.
• The study failed to support the original hypothesis.
• Scientists refined their hypotheses after the first round of results.
• A null hypothesis assumes no real effect exists until proven otherwise.
• The two hypotheses led to strikingly different predictions.
• Reviewers questioned whether the hypothesis was even testable.
• Multiple hypotheses were eliminated through careful experimentation.
• His hypothesis about the outbreak's origin was later confirmed.
• The team ruled out two of the three original hypotheses by midyear.
• Her hypothesis about the migration pattern was later confirmed by satellite tracking.
• The graduate student defended her hypothesis in front of a skeptical panel.
• The class was asked to form a testable hypothesis before starting the lab.