The Difference Between a Scientific Hypothesis, Theory, and Law

Let's address some common misconceptions about the basic concepts of science.
Maia Mulko

Nobody is exempt from misunderstanding scientific concepts and/or applying them incorrectly. Statistics from the National Science Board show that Americans scored an average of 5.6 over 9 true-or-false and multiple-choice science-related questions in 2016. Because of the low number of questions, the study is better at differentiating low and medium levels of knowledge than those with higher levels of knowledge. However, the results weren’t much different in previous studies, suggesting that Americans generally have had the same basic levels of science literacy since the beginning of the century.

In this context, we’d like to clear up and emphasize the distinctions between scientific hypothesis, theory, and law, and why you shouldn’t use these terms interchangeably. 

Hypothesis: the core of the scientific method

The scientific method is an empirical procedure that consists of systematic observation, measurement, and experiment, and the formulation, testing, and modification of hypotheses. It’s a process that’s meant to ensure that the collection of evidence, results, and conclusions are not biased by subjective views and can be repeated consistently by others.

Although there might be variations due to the requirements of each branch of science, the steps of the scientific method are more or less the same.

The scientific method often starts with an observation or asking a question, such as the observation of certain natural phenomena or asking why a particular phenomenon exists or why it occurs in a particular way.

Observation motivates a question and the question motivates an initial hypothesis. The initial hypothesis is a conjecture that works as a temporary answer to the question, formulated via induction on the basis of what’s been observed. 

To better understand this, let’s take the case of physician Ignaz Semmelweis. In mid-19th Century, he worked at the First Obstetrical Clinic of Vienna General Hospital, where 10% of women in labor died due to puerperal fever. Meanwhile, the Second Obstetrical Clinic had an average maternal mortality rate of 4%. Semmelweis asked himself why there was a discrepancy in mortality rates between the two clinics. 

Dr. Ignaz Semmelweis. Source: Wikimedia Commons

 Through observation, he determined and eliminated a number of differences between the two clinics. Because the techniques, climate, etc., were pretty much the same in both places, he ended up concluding that it had something to do with the health workers who helped women in labor. In the Second Clinic, births were attended only by midwives, while in the First Clinic, births were often attended by medical students who also performed autopsies. Semmelweis came up with the hypothesis that medical students spread “cadaveric contamination” in the First Clinic and this was causing the puerperal fever. 

He ordered all medical students to wash their hands with chlorinated lime after performing autopsies, and the mortality rate in the First Clinic decreased by 90%. 

Semmelweis is considered one of the early pioneers of antiseptic procedures.

This story doesn’t only demonstrate the importance of the initial hypothesis, but also the importance of testing it through experiments, field studies, observational studies, or other experimental work. In fact, this is the next step in the scientific method, and it’s essential to draw conclusions. 

Theory: the Why and How of natural phenomena

A scientific theory can be defined as a series of repeatedly tested and verified hypotheses and concepts. Scientific theories are based on hypotheses that are constructed and tested using the scientific method, and which may bring together a number of facts and hypotheses.

A theory synthesizes the discovered facts about phenomena in a way that allows scientists to formulate predictions and develop new hypotheses. For example, we can hypothesize the reasons why an animal looks or acts in a certain way based on Darwin’s theory of evolution. Or we can predict that antiseptics will prevent diseases if we take into account the germ theory. The confirmation of these hypotheses and predictions reinforces the theories they’re based on.

According to Charles Darwin, evolution happens by natural selection, which involves adaptation. Source: Maija Karala/Wikimedia Commons

For a theory to be valid, it must be testable, hold true for general tendencies and not only to specific cases, and it must not contradict verified pre-existing theories and laws. 

Law: the patterns of nature

In general, a scientific law is the description of an observed phenomenon. It doesn't explain why the phenomenon exists or what causes it. Laws can be thought of as the starting place, the point from where questions like "why" and "how" are asked.

For example, we can throw a ball under certain conditions and predict its movement by taking into account Newton’s laws of motion. These laws do not only involve several statements but also equations and formulas. However, while Newton's laws might mathematically describe how two bodies interact with each other, they don't explain what gravity is, or how it works. 

Newton's laws of motion are the most famous scientific laws. Source: Wikimedia Commons

Contrary to popular belief, scientific laws are not immutable. They must be universal and absolute to qualify as laws, but they can be corrected or extended to make them more accurate. For example, Euler’s laws of motion amplify Newton’s laws of motion to rigid bodiesand how gravity works was only understood in more detail when Albert Einstein developed the Theory of Relativity.

Common misconceptions about scientific laws, theories, and hypotheses

  • There is a hierarchy between laws, theories, and hypotheses:

    Some people think that hypotheses “evolve” into theories and theories “evolve” into laws. While a number of verified hypotheses can be included in a theory, it’s never only one. And theories do not turn into scientific laws because they’re simply different concepts. As stated above, theories explain phenomena and laws reflect patterns. 
  • Theories are not as “serious” as laws:

    Related to the hierarchy myth, this misconception comes from the colloquial use of the word “theory”, which is often applied as a synonym of “supposition”. But as we’ve mentioned earlier, scientific theories are exactly the opposite —not suppositions, but consistently verified facts that explain phenomena.
  • There is only one scientific method:

    The popularity of the simplified model of scientific inquiry has made some people think it’s the only way in which scientists do their thing. Although all the scientific methods follow certain rules and share some elements with others, the truth is that there are several models: the classical model (Aristotelian), the hypothetico-deductive model, the pragmatic model, etc.

You don’t have to be a scientist to understand scientific terms. In the information era, scientific concepts surround us, but even if access to knowledge is easier than ever nowadays, there are still a lot of misconceptions around. It's always better to be on the safe side and getting your facts straight.