Of all the methods used for research, the long term cohort study is the most seductive. You identify a group of people who share a date or place of birth or an experience of some kind, then study them over a period of time. Simple—at least in principle. In practice, because the best known cohort studies have also been very large, the logistical effort required to keep the show on the road is impressive in itself. So impressive that you can almost find yourself viewing any useful insights that emerge more as a bonus than the point of the exercise.

Next month sees the 65th birthday of the granddaddy of all cohort studies, the National Survey of Health and Development.1 Set up by James Douglas less than a year after the end of the second world war, it began with interviews of more than 13 000 mothers who had given birth in the United Kingdom during one week of March 1946. Concern over the low birth weight of babies born to less well-off mothers prompted a follow-up survey of more than 5000 of the original offspring. The project just kept on going. When the latest assessment began a few years ago its organisers were still in touch with around 3000 of the cohort.

The success of the project has inspired comparable work in several other countries from Finland to New Zealand, and also further cohort studies in the UK. These include the 1958 National Child Development Study, the 1970 British Cohort Study, and the Millennium Cohort started in 2000.2 But although birth studies of this kind are the most publicised use of the cohort approach, it can be …