Erica Gibson-Staneland, an anthropologist at Rollins College in Winter Park, Fla., has found that geophagy is more often found in cultures that do not practice dairying. Dairy products like milk and cheese would provide important dietary calcium -- when these are absent, pregnant women may seek other sources.
"It's about women lacking nutrients or women in impoverished conditions who don't have access to health care, adapting," said Gibson-Staneland.
"In Africa, they eat the dirt from termite mounds," she added, noting that the dirt and clays from termite mounds are rich in minerals.
Because not all clays are created equal, women who eat clay are very particular about which clays they consume.
"Everywhere that geophagy has been recorded, it's passed down that there's a certain location where the dirt is tastier or they know it to be cleaner," Gibson-Staneland said.
Joiner, who has eaten clay for over 20 years, refuses to eat certain clays because they contain sand or have a gritty taste. "Make sure you get the real stuff," she advised. "Some people just go out locally and get it. The store's chalk is not as good. I could actually taste the difference."
And most women who practice geophagy get their clays from sources other than the first few inches of topsoil, which have the most biological activity -- and the most bacteria, parasites and other pathogens.
"It's mostly subsurface stuff and I think that's probably less likely to be a source of infection," said Gerald N. Callahan, immunologist at Colorado State University in Fort Collins.
Callahan believes eating clay may be a way to build up the immune system during pregnancy. Citing what has been referred to as a "hygiene hypothesis," he noted that children raised in rural areas, especially on farms, have fewer allergies and autoimmune diseases than children raised in cities -- some researchers believe exposure to soil and other environmental impurities is the reason.
"It's possible that the [pregnant] woman would be strengthening her immune system at the same time she's going to turn antibodies to the child," said Callahan.
Another advantage to eating clay during pregnancy may be the calming effect it can have on the mother's gastrointestinal system, which can succumb to bouts of nausea and morning sickness.
Clays, especially white clays, are made of kaolin. Indeed, Rolaids, Maalox and other medicines recommended for nausea and stomach upset are filled with the same antacid compounds found in white kaolin clays.
Charles N. Maddox of Griffin, Ga., has for many years sold locally mined clay under the catchy brand name Down Home Georgia White Dirt. "It's just Kaopectate," Maddox said. "The old drunks, they used to get drunk and put it [clay] in water and drink it, then go to work. The old-timers and people in Louisiana and Mississippi -- they just love to eat that Mississippi mud."
Maddox's product, however, is not sold for human consumption, though he knows that some locals might be inclined to eat clay.
"We don't suggest that people eat it. We just bag it as a novelty," Maddox said.
Even avid fans of clay eating admit it can be a problem. There is a negative social stereotype associated with anyone who would consume dirt right out of the ground. Many of those familiar with the practice disdain "dirt-eaters" as poor, ignorant or malnourished.