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Biology · Chapter 2: The Scientific Method (Solving a Biological Problem)
Hypothesis and Deduction
What a hypothesis is, the features of a good hypothesis, if–then deductions, and inductive and deductive reasoning.
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01Concept
Once a biologist has a question, the next step is to suggest an answer. This suggested answer is called a hypothesis. A hypothesis is a possible explanation of an observation that can be tested by experiments.
Features of a good hypothesis
- It must be testable: an experiment or observation can show whether it is right or wrong.
- It must be consistent with the observations already made.
- It should be simple and clear, about one cause.
- It must be possible to prove it wrong (falsifiable). A hypothesis that no result could ever disprove is not scientific.
| Not a good hypothesis | A good hypothesis |
|---|---|
| Plants grow because nature wants them to. | Plants need light to make food, so they grow taller in sunlight. |
| Malaria is caused by bad luck. | Malaria is caused by a tiny organism in the blood. |
Deduction: "If…, then…"
A hypothesis cannot be tested directly. First the scientist works out what should happen if the hypothesis is true. This logical step is called deduction. Deductions are usually written as "If… then…" statements.
| Hypothesis | Deduction |
|---|---|
| Seeds need water to germinate. | If seeds need water, then seeds kept on dry cotton will not germinate, but seeds on wet cotton will. |
| Mosquitoes carry the cause of malaria. | If mosquitoes carry malaria, then people protected from mosquito bites will not get malaria. |
| Plants need light to make starch. | If light is needed, then a leaf kept in the dark for two days will give no blue-black colour with iodine. |
Inductive and deductive reasoning
| Inductive reasoning | Deductive reasoning |
|---|---|
| Goes from many particular observations to a general idea | Goes from a general idea to a particular prediction |
| Used to form a hypothesis | Used to test a hypothesis |
| Every mango leaf I have examined has veins, so all mango leaves have veins. | All birds have feathers. A sparrow is a bird. So a sparrow has feathers. |
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Real World Examples
Polio drops
Scientists hypothesised that a weakened virus given by mouth would protect children from polio. Their deduction, that vaccinated children would not catch polio, was tested in huge trials. Pakistan's polio campaigns are built on that tested hypothesis.
Locust swarms
When locusts attacked crops in Sindh and Balochistan in 2019–2020, scientists linked the swarms to unusual rains in the breeding areas. From that they deduced where swarms would come next and sent spraying teams there.
Your own life
If your phone does not switch on, you think "maybe the battery is empty" (hypothesis). Then: "if the battery is empty, then charging it will make the phone work" (deduction). Plugging it in is the experiment.
04Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Define hypothesis.Show answer
Answer
A hypothesis is a possible explanation of an observation that can be tested by experiments.
Q2.Give four features of a good hypothesis.Show answer
Answer
- It is testable.
- It agrees with the observations made.
- It is simple and clear.
- It can be proved wrong if it is wrong.
Q3.What is deduction? Give an example.Show answer
Answer
Deduction is working out what should happen if a hypothesis is true. It is written as an "if…then" statement.
Example: if seeds need water to germinate, then seeds on dry cotton will not germinate.
Q4.Differentiate between inductive and deductive reasoning.Show answer
Answer
| Inductive | Deductive |
|---|---|
| From particular observations to a general idea | From a general idea to a particular prediction |
| Used to form a hypothesis | Used to test a hypothesis |
Q5.Why should we say a hypothesis is "supported" rather than "proved"?Show answer
Answer
Because a later experiment may still show it to be wrong. One set of results can only support a hypothesis or reject it.
Long questions
Q6.A farmer observes that his tomato plants near a tall wall grow poorly. Write a hypothesis, a deduction and a simple experiment to test it.Show answer
Answer
- Hypothesis: the wall shades the plants, and they grow poorly because they get less sunlight.
- Deduction: if lack of sunlight is the cause, then tomato plants of the same kind grown in full sunlight, with the same soil and water, will grow taller and give more fruit.
- Experiment: grow 10 plants in the shade of the wall and 10 in full sun. Give them the same soil, water and fertiliser. Measure their height every week for six weeks and count the fruit.
- Conclusion: if the plants in the sun grow much better, the hypothesis is supported. If both groups grow equally poorly, it is rejected and a new hypothesis (for example, poor soil near the wall) is tested.