The statements made by Jake’s friends and family about what actions will help
him remain healthy (for example, his mother’s advice to wear a hat) are in some
part based on the advice-giver’s understanding of how our bodies resist colds.
Ideas about “how things work” are called hypotheses. Or, more formally, a hy-
pothesis is a proposed explanation for one or more observations. All of us gen-
erate hypotheses about the causes of some phenomenon based on our
understanding of the world (Figure 1.1). When Jake’s mom tells him to dress warmly in order to avoid colds, she is basing her advice on her belief in the fol lowing hypothesis: Becoming chilled makes an individual more susceptible to becoming ill.
The hallmark of science is that hypotheses are subject to rigorous testing. Therefore, scientific hypotheses must be testable—it must be possible to evaluate the hypothesis through observations of the measurable universe. Not all hypotheses are testable. For instance, the statement that “colds are generated by disturbances in psychic energy” is not a scientific hypothesis, since psychic energy cannot be seen or measured—it does not have a material nature. In addition, hypotheses that require the intervention of a supernatural force cannot be tested scientifically. If something is supernatural, it is not constrained by the laws of nature, and its behavior cannot be predicted using our current understanding of the natural world.
Scientific hypotheses must also be falsifiable, that is, able to be proved
false. The hypothesis that exposure to cold temperatures increases your sus-
ceptibility to colds is falsifiable, because we can imagine an observation would
cause us to reject this hypothesis (for instance, the observation that people ex-
posed to cold temperatures do notcatch more colds than people protected fromchills). However, hypotheses that are judgments, such as “It is wrong to cheat
on an exam,” are not scientific, since different people have different ideas about
right and wrong. It is impossible to falsify these types of statements.
The Logic of Hypothesis Testing
Of all the advice Jake has heard, he is inclined toward that given by his lab
partner. She insisted that taking vitamin C supplements was keeping her
healthy. Jake also recalls learning about vitamin C in his Human Nutrition class
last year. In particular, he remembers that:
1. Fruits and vegetables contain lots of vitamin C.
2. People with diets rich in fruits and vegetables are generally healthier
than people who skimp on these food items.
3. Vitamin C is known to be an anti-inflammatory agent, reducing throat
and nose irritation.
Given his lab partner’s experience and what he learned in class, Jake makes
the following hypothesis:
Consuming vitamin C decreases the risk of catching a cold.
This hypothesis makes sense. After all, Jake’s lab partner is healthy and Jake
has made a logical case for why vitamin C is good cold prevention. This cer-
tainly seems like enough information on which to base his decision about how
to proceed—he should start taking vitamin C supplements if he wants to avoid
future colds. However, a word of caution: Just because a hypothesis seems log-
ical does not mean that it is true.
Consider the ancient hypothesis that the sun revolves around Earth, as-
serted by Aristotle in approximately 350 B.C.This hypothesis was logical, based
on the observation that the sun appeared on the eastern horizon every day at
sunrise and disappeared behind the western horizon at sunset. For two thou-
sand years, this hypothesis was considered to be “a fact” by nearly all of West-
ern society. To most people, the hypothesis made perfect sense, especially since
the common religious belief in Western Europe was that Earth had been creat-
ed and then surrounded by the vault of heaven. It was not until the early sev-
enteenth century that this hypothesis was falsified as the result of observations
made by Galileo Galilei of the movements of Venus. Galileo’s work helped to
confirm Nicolai Copernicus’ more modern hypothesis that Earth revolves
around the sun.
So even though Jake’s hypothesis about vitamin C is perfectly logical, it
needs to be tested. Hypothesis testing is based on a process called deductive rea-
soningor deduction. Deduction involves making a specific predictionabout the
outcome of an action or test based on observable facts. The prediction is the re-
sult we would expect from a particular test of the hypothesis.
Deductive reasoning takes the form of “if/then” statements. Aprediction
based on the vitamin C hypothesis could be:
If vitamin C decreases the risk of catching a cold, then people who take vi-
tamin C supplements with their regular diets will experience fewer colds
than people who do not take supplements.
Deductive reasoning, with its resulting predictions, is a powerful method
for testing hypotheses. However, the structure of such a statement means that
hypotheses can be clearly rejected if untrue, but impossible to prove if they
are true (Figure1.2). This shortcoming is illustrated using the “if/then” state-
ment above.
Consider the possible outcomes of a comparison between people who sup-
plement with vitamin C and those who do not: People who take vitamin C sup-
plements may suffer through more colds than people who do not, they may
have the same number of colds as people who do not supplement, or supple-
menters may in fact experience fewer colds. What do these results tell Jake
about his hypothesis?
If people who take vitamin C have more colds, or the same number of colds
as those who do not supplement, the hypothesis that vitamin C alone provides
protection against colds can be rejected. But what if people who supplement
with vitamin C doexperience fewer colds? If this is the case, should Jake be out
proclaiming the news, “Vitamin C—A Wonder Drug that Prevents the Com-
mon Cold”? No, he should not. Jake needs to be much more cautious than that;
he can only say that he has supported and not disproven the hypothesis.
Why is it impossible to say that the hypothesis that vitamin C prevents
colds is true? Primarily because there could be other factors (that is, there are
alternative hypotheses) that explain why people with different vitamin-taking
habits are different in their cold susceptibility. In other words, demonstrating
the truth of the thenportion of a deductive statement does not guarantee that
the if portion is true.
Consider the alternative hypothesis that frequent exercise reduces sus-
ceptibility to catching a cold. Perhaps people who take vitamin C supple-
ments are more likely to engage in regular exercise than those who do not
supplement. What if the alternative hypothesis were true? If so, the predic-
tion that people who take vitamin C supplements experience fewer colds than
people who do not supplement would be true, but not because the original
hypothesis (vitamin C reduces the risk of cold) is true. Instead, people who
take vitamin C supplements experience fewer colds than people who do not
supplement because they are more likely to exercise, and it is exercise that
reduces cold susceptibility.
