A longitudinal study is a study that observes a subject or subjects over an extended period of time. They may run into several weeks, months, or years. An examples is the Up Series which has been going since 1963.
Longitudinal studies are deployed most commonly in psychology and sociology, where the intention is to observe the changes in the subject over years, across a lifetime, and sometimes, even across generations.
There have been several famous longitudinal studies in history. Some of the most well-known examples are listed below.
Examples of Longitudinal Studies
1. Up Series
Duration: 1963 to Now
The Up Series is a continuing longitudinal study that studies the lives of 14 subjects in Britain at 7-year intervals.
The study is conducted in the form of interviews in which the subjects report the changes that have occurred in their lives in the last 7 years since the last interview.
The interviews are filmed and form the subject matter of the critically acclaimed Up series of documentary films directed by Michael Apsted.
When it was first conceived, the aim of the study was to document the life progressions of a cross-section of British children through the second half of the 20th century in light of the rapid social, economic, political, and demographic changes occuring in Britain.
14 children were selected from different socio-economic backgrounds for the first study in 1963 in which all were 7 years old.
The latest installment was filmed in 2019 by which time the participants had reached 63 years of age.
The study noted that life outcomes of subjects were determined to a large extent by their socio-economic and demographic circumstances, and that chances for upward mobility remained limited in late 20th century Britain (Pearson, 2012).
2. Minnesota Twin Study
Duration: 1979 to 1990 (11 years)
Siblings who are twins not only look alike but often display similar behavioral and personality traits.
This raises an oft-asked question: how much of this similarity is genetic and how much of it is the result of the twins growing up together in a similar environment.
The Minnesota twin study was a longitudinal study that set out to find an answer to this question by studying a group of twins from 1979 to 1990 under the supervision of Thomas J Bouchard.
The study found that identical twins who were reared apart in different environments did not display any greater chances of being different from each other than twins that were raised in the same environment.
The study concluded that the similarities and differences between twins are genetic in nature, rather than being the result of their environment (Bouchard et. al., 1990).
3. Grant Study
Duration: 1942 – Present
The Grant Study is one of the most ambitious longitudinal studies. It attempts to answer a philosophical question that has been central to human existence since the beginning of time – what is the secret to living a good life? (Shenk, 2009).
It does so by studying the lives of 268 male Harvard graduates who are interrogated at least every two years with the help of questionnaires, personal interviews, and gleaning information about their physical and mental well-being from their physicians.
Begun in 1942, the study continues to this day.
The study has provided researchers with several interesting insights into what constitutes the human quality of life.
- It reveals that the quality of our relationships is more influential than IQ when it comes to our financial success.
- It suggests that our relationships with our parents during childhood have a lasting impact on our mental and physical well-being until late into our lives.
In short, the results gleaned from the study (so far) strongly indicate that the quality of our relationships is one of the biggest factors in determining our quality of life.
4. Terman Life Cycle Study
Duration: 1921 – Present
The Terman Life-Cycle Study, also called the Genetic Studies of Genius, is one of the longest studies ever conducted in the field of psychology.
Commenced in 1921, it continues to this day, over 100 years later!
The objective of the study at its commencement in 1921 was to study the life trajectories of exceptionally gifted children, as measured by standardized intelligence tests.
Lewis Terman, the principal investigator of the study, wanted to dispel the then-prevalent notion that intellectually gifted children tended to be:
- socially inept, and
- physically deficient
To this end, Terman selected 1528 students from public schools in California based on their scores on several standardized intelligence tests such as the Stanford-Binet Intelligence scales, National Intelligence Test, and the Army Alpha Test.
It was discovered that intellectually gifted children had the same social skills and the same level of physical development as other children.
As the study progressed, following the selected children well into adulthood and in their old age, it was further discovered that having higher IQs did not affect outcomes later in life in a significant way (Terman & Oden, 1959).
5. National Food Survey
Duration: 1940 to 2000 (60 years)
The National Food Survey was a British study that ran from 1940 to 2000. It attempted to study food consumption, dietary patterns, and household expenditures on food by British citizens.
Initially commenced to measure the effects of wartime rationing on the health of British citizens in 1940, the survey was extended and expanded after the end of the war to become a comprehensive study of British dietary consumption and expenditure patterns.
After 2000, the survey was replaced by the Expenditure and Food Survey, which lasted till 2008. It was further replaced by the Living Costs and Food Survey post-2008.
6. Millennium Cohort Study
Duration: 2000 to Present
The Millennium Cohort Study (MCS) is a study similar to the Up Series study conducted by the University of London.
Like the Up series, it aims to study the life trajectories of a group of British children relative to the socio-economic and demographic changes occurring in Britain.
However, the subjects of the Millenium Cohort Study are children born in the UK in the year 2000-01.
Also unlike the Up Series, the MCS has a much larger sample size of 18,818 subjects representing a much wider ethnic and socio-economic cross-section of British society.
7. The Study of Mathematically Precocious Youths
Duration: 1971 to Present
The Study of Mathematically Precocious Youths (SMPY) is a longitudinal study initiated in 1971 at the Johns Hopkins University.
At the time of its inception, the study aimed to study children who were exceptionally gifted in mathematics as evidenced from their Scholastic Aptitude Test (SAT) scores.
Later the study shifted to Vanderbilt University and was expanded to include children who scored exceptionally high in the verbal section of the SATs as well.
The study has revealed several interesting insights into the life paths, career trajectories, and lifestyle preferences of academically gifted individuals. For instance, it revealed:
- Children with exceptionally high mathematical scores tended to gravitate towards academic, research, or corporate careers in the STEM fields.
- Children with better verbal abilities went into academic, research, or corporate careers in the social sciences and humanities.
8. Baltimore Longitudinal Study of Aging
Duration: 1958 to Present
The Baltimore Longitudinal Study of Aging (BLSA) was initiated in 1958 to study the effects of aging, making it the longest-running study on human aging in America.
With a sample size of over 3200 volunteer subjects, the study has revealed crucial information about the process of human aging.
For instance, the study has shown that:
- The most common ailments associated with the elderly such as diabetes, hypertension, and dementia are not an inevitable outcome of growing old, but rather result from genetic and lifestyle factors.
- Aging does not proceed uniformly in humans, and all humans age differently.
9. Nurses’ Health Study
Duration: 1976 to Present
The Nurses’ Health Study began in 1976 to study the effects of oral contraceptives on women’s health.
The first commercially available birth control pill was approved by the Food and Drug Administration (FDA) in 1960, and the use of such pills rapidly spread across the US and the UK.
At the same time, a lot of misinformation prevailed about the perceived harmful effects of using oral contraceptives.
The nurses’ health study aimed to study the long-term effects of the use of these pills by researching a sample composed of female nurses.
Nurses were specially chosen for the study because of their medical awareness and hence the ease of data collection that this enabled.
Over time, the study expanded to include not just oral contraceptives but also smoking, exercise, and obesity within the ambit of its research.
As its scope widened, so did the sample size and the resources required for continuing the research.
As a result, the study is now believed to be one of the largest and the most expensive observational health studies in history.
10. The Seattle 500 Study
Duration: 1974 to Present
The Seattle 500 Study is a longitudinal study being conducted by the University of Washington.
It observes a cohort of 500 individuals in the city of Seattle to determine the effects of prenatal habits on human health.
In particular, the study attempts to track patterns of substance abuse and mental health among the subjects and correlate them to the prenatal habits of the parents.
From the examples above, it is clear that longitudinal studies are essential because they provide a unique perspective into certain issues which can not be acquired through any other method.
Especially in research areas that study developmental or life span issues, longitudinal studies become almost inevitable.
A major drawback of longitudinal studies is that because of their extended timespan, the results are likely to be influenced by epochal events.
For instance, in the Genetic Studies of Genius described above, the life prospects of all the subjects would have been impacted by events such as the Great Depression and the Second World War.
The female participants in the study, despite their intellectual precocity, spent their lives as home makers because of the cultural norms of the era. Thus, despite their scale and scope, longitudinal studies do not always succeed in controlling background variables.
Bouchard, T. J. Jr, Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: the Minnesota study of twins reared apart. Science, 250 (4978), 223–228. doi: https://doi.org/10.1126/science.2218526
Pearson, A. (2012, May) Seven Up!: A tale of two Englands that, shamefully, still exist The Telegraph https://www.telegraph.co.uk/comment/columnists/allison-pearson/9269805/Seven-Up-A-tale-of-two-Englands-that-shamefully-still-exist.html
Shenk, J.W. (2009, June) What makes us happy? The Atlantic https://www.theatlantic.com/magazine/archive/2009/06/what-makes-us-happy/307439/
Terman, L. M. & Oden, M. (1959). The Gifted group at mid-Life: Thirty-five years’ follow-up of the superior child. Genetic Studies of Genius Volume V. Stanford University Press.