Hypothesis Plural Form: Rule, Examples, and Mistakes

The plural form of “hypothesis” is “hypotheses.” This change follows the Greek-based rule for words ending in “-sis,” where the “-sis” becomes “-ses” in the plural. So, one hypothesis becomes two or more hypotheses. This is not a simple English plural like adding “-s” or “-es,” which is why it often causes confusion. This guide explains the rule, gives you clear examples, shows you common mistakes, and helps you use “hypotheses” correctly in your writing and speaking.

Quick Answer: Hypothesis or Hypotheses?

• Singular: hypothesis (one idea or proposed explanation)
• Plural: hypotheses (more than one idea or proposed explanation)
• Rule: Change “-sis” to “-ses” (like “thesis” to “theses” and “crisis” to “crises”)

Use “hypothesis” when you are talking about a single theory. Use “hypotheses” when you are discussing multiple theories or proposed explanations.

Understanding the Rule for Hypothesis

The word “hypothesis” comes from Greek, and English keeps the original Greek plural pattern for many academic and scientific words. The pattern is straightforward: if a word ends in “-sis,” you change that ending to “-ses” to make it plural. This rule applies to words like “analysis” (analyses), “diagnosis” (diagnoses), and “parenthesis” (parentheses).

Because “hypothesis” is a common term in research, science, and academic writing, knowing its correct plural form is essential for clear communication. Using “hypothesises” or “hypothesis” as a plural is a common error that can make your writing look less careful.

Formal vs. Informal Tone

In formal academic writing, you will almost always use “hypotheses” when referring to more than one. For example, in a research paper, you might write: “The study tested three hypotheses.” In informal conversation, people sometimes say “hypothesis” for both singular and plural, but this is not correct in careful English. If you are writing an email to a professor or a report for work, always use “hypotheses” for the plural.

Email and Conversation Context

In a professional email, you might write: “I have attached the data for the two hypotheses we discussed.” In a conversation with a colleague, you could say: “We need to test both hypotheses before the meeting.” The word “hypotheses” sounds natural and precise in both contexts.

Comparison Table: Hypothesis vs. Hypotheses

Natural Examples

Here are examples of how “hypothesis” and “hypotheses” are used in real writing and speech:

• Singular: “The researcher proposed a new hypothesis about climate change.”
• Plural: “Several hypotheses were considered before the experiment began.”
• Singular: “My hypothesis is that the plant will grow faster with more sunlight.”
• Plural: “The students developed two hypotheses for their science project.”
• Singular: “This hypothesis explains the observed results.”
• Plural: “The paper reviews the main hypotheses in the field of genetics.”

Common Mistakes

Even advanced English learners make mistakes with “hypothesis.” Here are the most frequent errors and how to fix them:

Mistake 1: Using “Hypothesis” as a Plural

Incorrect: “We have three hypothesis to test.”
Correct: “We have three hypotheses to test.”
Why: “Hypothesis” is singular. For more than one, you must use “hypotheses.”

Mistake 2: Adding “-es” to Make “Hypothesises”

Incorrect: “The scientist proposed several hypothesises.”
Correct: “The scientist proposed several hypotheses.”
Why: “Hypothesis” does not follow the regular plural rule. The correct plural is “hypotheses,” not “hypothesises.”

Mistake 3: Confusing “Hypothesis” with “Thesis”

Incorrect: “The student wrote a hypothesis for their degree.” (when they mean a long paper)
Correct: “The student wrote a thesis for their degree.”
Why: A “thesis” is a long academic paper, while a “hypothesis” is a proposed explanation. Both have the same plural rule (“theses” and “hypotheses”), but their meanings are different.

Better Alternatives and When to Use Them

Sometimes, you might want to use a different word to avoid repetition or to be more specific. Here are some alternatives:

• For “hypothesis” (singular): theory, assumption, proposition, supposition, postulate. Use “theory” when the idea is well-supported, and “assumption” when it is taken for granted.
• For “hypotheses” (plural): theories, assumptions, propositions, suppositions, postulates. These work well in academic writing when you want to vary your language.

When to use “hypothesis” vs. “theory”: In science, a hypothesis is a testable prediction, while a theory is a well-substantiated explanation. In everyday English, “theory” is often used more loosely. For example: “I have a theory about why the meeting was cancelled” is common, but “I have a hypothesis about why the meeting was cancelled” sounds more formal and precise.

Mini Practice: Test Your Knowledge

Choose the correct word to complete each sentence. Answers are below.

1. The researcher developed a new _____ to explain the results. (hypothesis / hypotheses)
2. We tested several _____ during the experiment. (hypothesis / hypotheses)
3. One of the _____ was proven incorrect. (hypothesis / hypotheses)
4. The paper compares two competing _____ in the field. (hypothesis / hypotheses)

Answers:

1. hypothesis (singular, one idea)
2. hypotheses (plural, more than one idea)
3. hypotheses (plural, “one of the” refers to a group, so plural is needed)
4. hypotheses (plural, “two” requires the plural form)

Frequently Asked Questions (FAQ)

1. Is “hypotheses” the only correct plural of “hypothesis”?

Yes, “hypotheses” is the only standard plural form. “Hypothesises” is not correct in standard English. Some very informal speech might use “hypothesis” for both, but this is not accepted in writing or formal contexts.

2. How do you pronounce “hypotheses”?

“Hypotheses” is pronounced /haɪˈpɒθəsiːz/ (hy-POTH-uh-seez). The last syllable sounds like “sees,” not “sis.” The singular “hypothesis” is pronounced /haɪˈpɒθəsɪs/ (hy-POTH-uh-sis).

3. Can I use “hypothesis” in everyday conversation?

Yes, but it sounds formal. In casual conversation, people often say “guess” or “theory” instead. For example: “My guess is that it will rain” is more natural than “My hypothesis is that it will rain” in everyday speech. However, in academic or professional settings, “hypothesis” is appropriate.

4. What is the difference between “hypothesis” and “prediction”?

A hypothesis is a proposed explanation that can be tested. A prediction is a specific statement about what will happen in the future, often based on a hypothesis. For example: “The hypothesis is that caffeine improves focus. The prediction is that people who drink coffee will score higher on the test.”

Final Tips for Using “Hypothesis” and “Hypotheses”

To remember the correct plural, think of other “-sis” words you know, like “analysis” and “crisis.” If you can remember that “analyses” is the plural of “analysis,” you can apply the same rule to “hypotheses.” Practice by writing sentences with both forms. For example, write one sentence with “hypothesis” and one with “hypotheses” each day for a week. This will help the correct forms feel natural.

If you are unsure which form to use, ask yourself: “Am I talking about one idea or more than one?” If it is one, use “hypothesis.” If it is two or more, use “hypotheses.” This simple check will help you avoid the most common mistakes.

For more help with plural forms, visit our Plural Spelling Rules section. You can also check our Common Plural Forms for other tricky words. If you have questions, see our FAQ page or contact us.