Can stress turn your hair gray? Here's a story on gray hair research, written by a New York Times science journalist. In our culture, we certainly believe that gray hair can be caused (in some cases at least) by stress. There is clear evidence that gray hair is inherited genetically, apparently stress plays a role, too
Here's how the journalist introduces the first study:
In past studies, researchers have asked participants to fill out questionnaires about their hair color and stress levels, and then the scientists would see if they could link them.
In one study published in 2016, for instance, scientists surveyed more than 1,100 young Turkish adults and found that the 315 who reported prematurely graying hair had higher stress levels than those who didn’t.
a) Identify the two variables in this study.
b) Were the variables manipulated or measured?
c) Was this a correlational or experimental study?
d) Can the results support a claim that stress causes premature gray hair? Why or why not? Apply the three criteria for causation: Covariance, Temporal Precedence, and Internal Validity.
For question d above, you should have responded no; although this study did show a relationship between premature graying and perceived stress, the study doesn't establish temporal precedence since both variables were measured at the same time. And, it's possible some third variable is responsible--for example, maybe older participants have both more stress and more gray hair. (Note that even though genetics is the biggest predictor of premature gray hair, genetics isn't a confound here because the genes for gray hair don't also predict perceived stress.)
To show causation, it helps to have an experiment. Here's an example:
But a mouse study published in 2020 took the research a step forward. In it, researchers stressed mice in various ways, including by injecting them with a chili-pepper-like chemical that induced a “fight-or-flight” response. This caused them to release the stress hormone norepinephrine, which, in turn, depleted their hair follicles of the stem cells involved with adding pigment to mouse fur. The hair then grew in gray.
But studies on this topic are challenging to perform on people, because researchers can’t ethically induce artificially high stress responses in humans like they can in animals or cells, Dr. Hsu said.
e) What is the independent variable in this study?
f) What were the dependent variables (there were two)?
g) If you've read Chapter 9, you'll recognize this as a mediator argument: "[The chili-pepper chemical] caused them to release the stress hormone norepinephrine, which, in turn, depleted their hair follicles of the stem cells involved with adding pigment to mouse fur. The hair then grew in gray." Can you draw this mediator argument using boxes for each variable?
Here's one more study on humans that was better able to manage temporal precedence:
One small human study published in 2021 still advanced the narrative: Researchers plucked various strands of hair from 14 volunteers who had at least some graying. Several of the strands were fully gray, some were partially gray and some hadn’t grayed at all. The scientists then created high-resolution digital images of the hairs and calculated when each strand went gray using estimates of how quickly hair grows.
They also asked the participants to plot out stressful experiences from the past year on a timeline, and rank them from least to most distressing. The researchers found that when a strand turned gray frequently corresponded with the most stressful moment of that volunteer’s previous year.
h) There seem to be two variables in this study. Can you name them and their levels?
i) Was this a correlational or experimental study?
Taken together, the three studies here can be seen as an example of "pattern and parsimony." Some of the studies are correlational, others are experimental. Some are on humans and some are on animals. But they all seem to parsimoniously support an argument that stress precedes gray hair. What do you think?