Short answer: scientific studies contradict each other because they may ask slightly different questions, use different methods, study different populations, measure different outcomes, or analyse uncertain data in different ways.
Contradiction in science is frustrating, but it is not always a sign that science is broken. Research often moves from rough early evidence toward clearer understanding. Early studies may be small. Later studies may be larger. A laboratory result may not appear in the real world. A result in one group of people may not apply to another group.
Common reasons studies disagree
One major reason is design. A randomized controlled trial, an observational cohort study, a survey, and a case report do not answer questions in the same way. Each design has strengths and weaknesses. If two studies use different designs, apparent disagreement may simply reflect different kinds of evidence.
Another reason is measurement. A study about “exercise” may mean daily walking, high-intensity training, or self-reported activity. A study about “academic achievement” may use exam scores, teacher reports, attendance, or long-term outcomes. When the measurement changes, the answer can change too.
Statistics also matter. Small studies are more vulnerable to random noise. Studies testing many possible outcomes can find apparently significant results by chance. Selective reporting can make the published literature look more certain than the underlying evidence really is.
How readers should respond
Do not judge an issue by one isolated paper. Look for systematic reviews, meta-analyses, large studies, replications, and expert consensus statements. Ask whether the studies truly contradict each other or whether headlines have simplified them beyond recognition.
Scientific disagreement is not automatically a defect. It can be the mechanism by which weak claims are challenged and stronger claims emerge. The important question is whether the field is moving toward better explanations, better data, and clearer limits.