Key Takeaways
- A longitudinal study tracks people over a long time to see how things change.
- These studies can show how things like exercise might affect health as we age.
- Longitudinal studies can be costly and time-consuming to conduct.
A longitudinal study follows what happens to selected variables over an extended time. Psychologists use the longitudinal study design to explore possible relationships among variables in the same group of individuals over an extended period.
The Typical Longitudinal Study
Once researchers have determined the study’s scope, participants, and procedures, most longitudinal studies begin with baseline data collection. In the days, months, years, or even decades that follow, they continually gather more information so they can observe how variables change over time relative to the baseline.
For example, imagine that researchers are interested in the mental health benefits of exercise in middle age and how exercise affects cognitive health as people age. The researchers hypothesize that people who are more physically fit in their 40s and 50s will be less likely to experience cognitive declines in their 70s and 80s.
Longitudinal vs. Cross-Sectional Studies
Longitudinal studies, a type of correlational research, are usually observational, in contrast with cross-sectional research. Longitudinal research involves collecting data over an extended time, whereas cross-sectional research involves collecting data at a single point.
To test this hypothesis, the researchers recruit participants who are in their mid-40s to early 50s. They collect data related to current physical fitness, exercise habits, and performance on cognitive function tests. The researchers continue to track activity levels and test results for a certain number of years, look for trends in and relationships among the studied variables, and test the data against their hypothesis to form a conclusion.
Examples of Early Longitudinal Study Design
Examples of longitudinal studies extend back to the 17th century, when King Louis XIV periodically gathered information from his Canadian subjects, including their ages, marital statuses, occupations, and assets such as livestock and land. He used the data to spot trends over the years and understand his colonies’ health and economic viability.
In the 18th century, Count Philibert Gueneau de Montbeillard conducted the first recorded longitudinal study when he measured his son every six months and published the information in “Histoire Naturelle.”
The Genetic Studies of Genius (also known as the Terman Study of the Gifted), which began in 1921, is one of the first studies to follow participants from childhood into adulthood. Psychologist Lewis Terman’s goal was to examine the similarities among gifted children and disprove the common assumption at the time that gifted children were “socially inept.”
Types of Longitudinal Studies
Longitudinal studies fall into three main categories.
- Panel study: Sampling of a cross-section of individuals
- Cohort study: Sampling of a group based on a specific event, such as birth, geographic location, or experience
- Retrospective study: Review of historical information such as medical records
Benefits of Longitudinal Research
A longitudinal study can provide valuable insight that other studies can’t. They’re particularly useful when studying developmental and lifespan issues because they allow glimpses into changes and possible reasons for them.
For example, some longitudinal studies have explored differences and similarities among identical twins, some reared together and some apart. In these types of studies, researchers tracked participants from childhood into adulthood to see how environment influences personality, achievement, and other areas.
Because the participants share the same genetics, researchers chalked up any differences to environmental factors. Researchers can then look at what the participants have in common and where they differ to see which characteristics are more strongly influenced by either genetics or experience. Note that adoption agencies no longer separate twins, so such studies are unlikely today. Longitudinal studies on twins have shifted to those within the same household.
Potential Pitfalls
As with other types of psychology research, researchers must take into account some common challenges when considering, designing, and performing a longitudinal study.
Cost
Longitudinal studies require time and are often quite expensive. Because of this, these studies often have only a small group of subjects, which makes it difficult to apply the results to a larger population.
Selective Attrition
Participants sometimes drop out of a study for any number of reasons, like moving away from the area, illness, or simply losing motivation. This tendency, known as selective attrition, shrinks the sample size and decreases the amount of data collected.
If the final group no longer reflects the original representative sample, attrition can threaten the validity of the experiment. Validity refers to whether or not a test or experiment accurately measures what it claims to measure. If the final group of participants doesn’t represent the larger group accurately, generalizing the study’s conclusions is difficult.
The World’s Longest-Running Longitudinal Study
Lewis Terman aimed to investigate how highly intelligent children develop into adulthood with his “Genetic Studies of Genius.” Results from this study were still being compiled into the 2000s. However, Terman was a proponent of eugenics and has been accused of letting his own sexism, racism, and economic prejudice influence his study and of drawing major conclusions from weak evidence. However, Terman’s study remains influential in longitudinal studies. For example, a recent study found new information on the original Terman sample, which indicated that men who skipped a grade as children went on to have higher incomes than those who didn’t.
Frequently Asked Questions
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What is the difference between a longitudinal study and a cross-sectional study?
A longitudinal study follows up with the same sample (i.e., group of people) over time, whereas a cross-sectional study examines one sample at a single point in time, like a snapshot.
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How long can a longitudinal study last?
A longitudinal study can occur over any length of time, from a few weeks to a few decades or even longer.
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How many participants are needed for a longitudinal study?
That depends on what researchers are investigating. A researcher can measure data on just one participant or thousands over time. The larger the sample size, of course, the more likely the study is to yield results that can be extrapolated.
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Piccinin AM, Knight JE. History of longitudinal studies of psychological aging. Encyclopedia of Geropsychology. 2017:1103-1109. doi:10.1007/978-981-287-082-7_103
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Terman L. Study of the gifted. In: The SAGE Encyclopedia of Educational Research, Measurement, and Evaluation. 2018. doi:10.4135/9781506326139.n691
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Sahu M, Prasuna JG. Twin studies: A unique epidemiological tool. Indian J Community Med. 2016;41(3):177-182. doi:10.4103/0970-0218.183593
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Almqvist C, Lichtenstein P. Pediatric twin studies. In: Twin Research for Everyone. Elsevier; 2022:431-438.
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Warne RT. An evaluation (and vindication?) of Lewis Terman: What the father of gifted education can teach the 21st century. Gifted Child Q. 2018;63(1):3-21. doi:10.1177/0016986218799433
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Warne RT, Liu JK. Income differences among grade skippers and non-grade skippers across genders in the Terman sample, 1936–1976. Learning and Instruction. 2017;47:1-12. doi:10.1016/j.learninstruc.2016.10.004
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Wang X, Cheng Z. Cross-sectional studies: Strengths, weaknesses, and recommendations. Chest. 2020;158(1S):S65-S71. doi:10.1016/j.chest.2020.03.012
Additional Reading
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Caruana EJ, Roman M, Hernández-Sánchez J, Solli P. Longitudinal studies. J Thorac Dis. 2015;7(11):E537-E540. doi:10.3978/j.issn.2072-1439.2015.10.63
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