There is an inherent story-telling appeal to the longitudinal study. The Up Series, a British documentary that revisits the same children every seven years of their lives, demonstrates the entrenched nature of the class system in England. Unfolding before the audience’s eyes, each child’s life follows a course seemingly predestined by his or her circumstances of birth.
The longitudinal study’s main limitation is that it is not an experiment, where one factor is intentionally manipulated by a scientist. To draw more solid conclusions, babies in the documentary series would have been randomly assigned to families along the socio-economic spectrum. In recognition of basic ethical standards and to be true to life, studies like these are based on observation.
The 75-year Study of Adult Development at Harvard Medical School followed hundreds of American men who came of age during the Second World War. For this generation, alcoholism was the largest cause of divorce, and smoking was the greatest contributor to early death. Even levers of life happiness—overwhelmingly determined by warm relationships—could be teased out from this study.
The longitudinal study has great power because it can compare later values with the same person’s baseline state. Many of the established baselines in medicine have been much better studied in men than in women, who tend to have lower blood pressure and higher heart rates, among other differences. What is average for the general population may represent an alarming shift for any given person.
The trouble is, long-term observation is only possible if it is as unobtrusive as possible. The moment monitoring becomes burdensome for subjects, the results may not be representative of normal life—and subjects may start disappearing from a study.
Take blood pressure, which is particularly hard to observe in real time over the course of months in a healthy person. “A major hurdle to the kind of longitudinal monitoring we would like to do is reluctance to use the monitor,” says Cornelissen. “We are looking for a non-invasive way to monitor continuously for a week or longer that does not use uncomfortable cuffs.”
One potential avenue would be a flexible electronic circuit like the one John Rogers, a materials scientist at Northwestern University, has developed. Dubbed “epidermal electronics,” the device sticks like a temporary tattoo over a vein or artery on the forearm. It delivers continuous heat, and based on the rate of cooling, infers the amount of blood flow carrying that heat away.
The contemporary craze for health-monitoring smart-phone apps has prepared a wide segment of society for tracking their medical data through time. Cornelissen is hopeful that a long-term crowd-sourcing approach to blood pressure measurement could contribute much more to our understanding than one-off studies.