At Johns Hopkins University's Laboratory for Child Development, researchers learn about how babies and young children perceive and learn about their environment.

Roughly 13 minutes into a science experiment involving sets of blocks, Dorian Larkins, who at that moment is two weeks shy of his third birthday, has had enough. He would rather play with the trains in the waiting room, and he says he has to pee.

But at his mother's urging, he soldiers on for another five minutes or so until the session, which looks at toddler memory, wraps up. His mom believes it's imperative that Dorian participate in the work of the Laboratory for Child Development at Johns Hopkins University — but less so for her son's sake than for the researcher's.

"Being an African-American child, it's really important to have him in the study, to have some diversity," said Mona Larkins, 42.

She's right. But it turns out that it's not so much race and ethnicity — which Larkins values — that many labs need to improve upon these days. It's socioeconomic diversity.

Recognizing that, child development researchers are varying their recruitment techniques and trying to build bridges with poor communities by bringing their labs to the people, exploring possible school partnerships and relying on an old standby: cash payments.

"You really have to convince them that there's something in it for them, that you're not just getting data and running," said Lonnie R. Sherrod, executive director of the Society for Research in Child Development.

Lower-income families, who are frequently in the racial minority, often don't have the time, the incentive or even the transportation to participate in research, scientists said. They also don't necessarily trust the researchers or understand the point of the work, which may not yet have a practical application.

But their absence has consequences, particularly in the field of child development research, which aims to help kids grow. Low-income kids are often the most in need of help, and they're a big group: Nearly half of the population of children under age 6 lives in a low-income household, according to the National Center for Childhood Poverty.

Leaving them out of child development research means that the findings can't be generalized for everyone, and it might miss the specific needs of the poorer kids, who may learn differently because of their environment.

"It's easier for people with means to … come into the lab, and so it tends to be middle-, upper-middle-class families" in the studies, said Justin Halberda, who co-directs the Hopkins lab with his wife, Lisa Feigenson. Founded in 2004, the lab investigates what infants and children know, particularly with regard to numbers.

"There's no real racial difference," Halberda said. "It's really an economic difference."

But "people are more sensitive to ethnicity than they are to social class," and that's where they often put their focus when designing experiments, Sherrod said. "I think that's a mistake because I actually think social class is the more important variable in this country. It's an underappreciated variable."

For decades, women and minorities were frequently left out of clinical research. The disparity drew attention in the 1980s, and in 1993, the National Institutes of Health was mandated by law to include both groups in its research.

The federal agencies now require that scientists provide a racial and gender breakdown of their work in order to receive grant funds. But they don't typically ask about income, and consequently, neither do many researchers, some said, unless the study is specifically concerned with it.

At the University of Maryland's Child Development Lab, scientists focus on having a racially diverse sample, knowing that income variability is challenging to attain.

"College Park is not an easy place for families who don't have their own car and have to use public transportation to be able to get to," said Director Nathan A. Fox. "If you have a baby and are using a stroller or carriage and coming with other children, it becomes difficult."

Angeline Lillard, director of the Early Development Lab at the University of Virginia in Charlottesville, said her center recruits by holding an annual fair — complete with clowns and face painting — and by attending local farmers' markets and book fairs. "Basically any place where there are lots of young children," she said.

"What we don't see enough of is low-income families," she said. "Middle income and well-educated people of all races will tend to go to the kinds of events where we recruit. It really seems to be more about income and resources."

For a project that began three years ago in Hartford, Conn., looking specifically at the effect of a Montessori-style education on a low-income population, Lillard's lab initially had a hard time recruiting.

"Parents didn't even understand what the purpose was of doing the research," Lillard said. "There is some element of getting the value of it, which if you didn't go to college and take these courses, you might not know. And I think they are perhaps more consumed with how they can help their own child personally to survive and get through, such that the idea of sitting back and helping science more generally — that's kind of a luxury."