NPR reports on some research linking the type of bacteria that live in our digestive systems with the emotions that we experience. They report several studies, some of which are correlational and others are experimental. In the following excerpts,
a. Decide if the study is correlational or experimental.
b. Identify the variables in each example.
c. Is the researcher or journalist making a causal claim in the excerpt? If so, is the causal claim justified?
Here's Excerpt 1:
[Scientist Dr. Emeran Mayer] is working on just that, doing MRI scans to look at the brains of thousands of volunteers and then comparing brain structure to the types of bacteria in their guts. He thinks he already has the first clues of a connection, from an analysis of about 60 volunteers.
Mayer found that the connections between brain regions differed depending on which species of bacteria dominated a person's gut. That suggests that the specific mix of microbes in our guts might help determine what kinds of brains we have — how our brain circuits develop and how they're wired.
Here's Excerpt 2:
... other researchers have been trying to figure out a possible connection by looking at gut microbes in mice. There they've found changes in both brain chemistry and behavior. One experiment involved replacing the gut bacteria of anxious mice with bacteria from fearless mice.
"The mice became less anxious, more gregarious," says Stephen Collins of McMaster University in Hamilton, Ontario, who led a team that conducted the research.
It worked the other way around, too — bold mice became timid when they got the microbes of anxious ones. And aggressive mice calmed down when the scientists altered their microbes by changing their diet, feeding them probiotics or dosing them with antibiotics.
Here's Except 3:
One team of researchers in Baltimore is testing a probiotic to see if it can help prevent relapses of mania among patients suffering from bipolar disorder.
Mayer also has been studying the effects of probiotics on the brain in humans. Along with his colleague Kirsten Tillisch, Mayer gave healthy women yogurt containing a probiotic and then scanned their brains. He found subtle signs that the brain circuits involved in anxiety were less reactive, according to a paper published in the journal Gastroenterology.
Suggested answers to Excerpt 1:
a. This is a correlational study.
b. One variable is the kind of microbes in each volunteer's gut, a measured variable. The other variable is brain structure, also measured.
c. The statement, "the specific mix of microbes in our guts might help determine what kinds of brains we have" is a causal statement because of the verb, "determine." However, a correlational study can't support this causal statement. Because both variables were measured at the same time, we can't tell if the brain structure affected the gut microbes, or if the microbes affected the brain structures. It's also possible that some outside variable (Diet? Experience?) affected both at the same time.