Living in the San Francisco Bay Area for the past few years, I’ve gotten used to lots of things that would probably seem strange in other cities. Commuting on a unicycle? Sure. Rampant midday nudity? Everywhere. Vegan dinner fundraiser for your Burning Man art car? Of course. So I hardly bat an eye when a 4-year-old says, “My favorite food is edamame.”

As a developmental psychologist, I test children to learn basic facts about kids, such as how they learn language, navigate social interactions, and gain knowledge. These things seem like they should work about the same way for any young human. But there is growing evidence that the timing and efficiency with which children learn these general skills vary a lot based on experience. A huge amount of a child’s early life experience is determined by the family’s socioeconomic status—how wealthy and educated the child’s parents are. The edamame-loving professors’ kids I’ve been testing are unlikely to be representative of an average child, or even an average American child.

There’s a term to describe the types of people who participate in most social science studies: WEIRD. They are weird in the sense that they are unusual compared with most of the world’s population, but WEIRD is also an acronym describing the Western, educated, industrialized, rich, and democratic culture they come from.* A trio of psychology professors coined this term in a 2010 paper, pointing out that fields studying human behavior often use participants who are “Western, and more specifically American, undergraduates.”

The WEIRD issue has been talked about in psychology for many years, but there’s been little progress in addressing it. The problem is arguably even worse in developmental psychology than in other subfields. While many social science researchers recruit participants from a pool of hundreds of undergrads who must complete studies for course credit, there are no comparably large pools of children to test.

My lab mates and I recruit kids by sending mass mailings to addresses we get from state birth records, and interested parents call, email, or sign up online for an appointment. We also get a good number of participants who are friends of previous participants. Parents visit our lab at the University of California at Berkeley campus during the workweek. We don’t keep demographic records about our participants or their families, but a lot of cues suggest they’re WEIRD: their cars, iPads, fancy strollers, clothes, and the parents’ small talk about their education (“Oh, I went to Berkeley for undergrad!”). People who have the time and job flexibility to bring their kids to a lab in the middle of the workday tend to be fairly well off.

Another place we find children to test is the university’s preschools, where the kids are even WEIRDer than those who visit our lab. (The edamame kid went to preschool here.) Monthly tuition costs between $1,500 and $2,000, and the parents are highly educated, university-affiliated faculty, staff, researchers, or students. These kids know the drill with grad students’ studies: When a new person comes to class and asks if you want to “play a game,” that person is a researcher. While other children play “House” or “Doctor,” these Berkeley kids have been known to play a game called “Research.” One child holds a clipboard and asks other children to “play a game” while the child observes them and pretends to jot down notes. Some of these children have told me about their international travels, and several of the 3-year-olds have told me they can read.

Stanford University psychology professor Anne Fernald recognized the same WEIRD bias in the families that participate in her studies. Stanford’s campus is in the heart of Silicon Valley; tech giants Google and Facebook are less than five miles away. Her lab’s testing pool included some of the wealthiest and most highly educated people in the United States.

Fernald and her colleagues tried to recruit a more diverse sample for their studies but had little success. She suspects a combination of factors prevented less affluent families from visiting the lab. “The university was an alien place,” she says. “It’s intimidating, it’s costly, you need to find parking, you don’t know English, you don’t have the nanny at home to take care of the other child.” Fernald added that many of the non-English-speaking families in the area are undocumented and may have been hesitant to participate in any university programs that could expose their status.

In 2006, the lab decided that instead of asking less affluent families to come to it, the lab needed to go to them. Fernald rented a five-bedroom house in an East Palo Alto neighborhood where the average annual income is less than $25,000 and only a few parents have more than a high school education.

A postdoc, Nereyda Hurtado, lived in the house full time so that she would become a part of the community. Things were slow going while the neighbors tried to figure out what the strange research house was all about—did the people there care about immigration status? Were there doctors who could help? (Hurtado and other researchers did give parents information about agencies to contact for help with health or social service issues.) “Gradually, word spread that it was a safe place to come,” Fernald said. “We became a trusted resource in the neighborhood and soon, people were asking if they could participate.”

At the house, 18-, 24-, and 30-month-old children were tested on a task called “looking while listening.” It measures infants’ word processing skills in real time, and it’s beautifully simple. Children are seated in front of two pictures—say, a dog and a baby—and are asked to find one of the items: “Where’s the dog?” Researchers time how long it takes children to look toward the appropriate picture. Faster looks indicate faster cognitive processing, and quickness is correlated with positive long-term outcomes, such as better scores on third-grade standardized tests.

Fernald and her colleagues found that children’s speed in the looking-while-listening task could be predicted by their socioeconomic status, or SES. Though nearly all mothers in the East Palo Alto sample had less than a high school education, there was enough of a range within the sample to show that maternal education level was correlated with faster response times. Researchers often make claims about the skills children possess at certain ages, but this study highlights that individual children can vary widely and that these claims may not generalize to all children in the same way. If researchers studying so-called general skills in campus labs “were to do the same experiment two miles away, they would very likely find different results or later emergence of that ability,” Fernald says.

Fernald and her team had a hard time figuring out how much of these differences were due to SES, or whether ethnicity, culture, or native language may have also contributed. To tease apart those factors, the lab needed to compare participants who differed in only one of those characteristics rather than all four. “Our answer to that—a conscious, difficult, and expensive answer—was that we need to open another community lab in a low-SES, English-speaking community,” said Fernald.

The San Francisco Bay Area is notorious for its high cost of living, so Fernald and her researchers needed to get away. They outfitted a 31-foot RV with a lab space and traveled to Mendocino, Lake, and Shasta counties in Northern California, where they tested children from low-income English-speaking families living at the poverty line. (The median per-capita income was $23,900, comparable to the East Palo Alto sample.)

Children participated in the looking-while-listening task at two ages: once at 18 months old, and again at 24 months old. Both the low-SES Northern California children and children from a high-SES group tested at Stanford improved between sessions, but at both ages, high-SES children were quicker to match a word with a picture. In fact, the high-SES kids’ speed at 18 months was roughly the same as the low-SES kids’ speed at 24 months. In other words, it appears that low-SES kids are roughly six months behind their high-SES counterparts.

Other researchers are also taking a systematic look at the effects of SES on children. Kimberly Noble, a developmental cognitive neuroscientist at Columbia University, has found that SES predicts differences in brain structure and function, which are related to language learning, memory, and self-control. For example, Noble has found that low-SES children tend to have a smaller hippocampus, a part of the brain used in the formation of memories, and show different levels of neural activity in language areas of the brain while reading. “From a scientific perspective, we really need to be taking [children’s] background into account,” she says. “SES is ignored in a lot of cognitive and developmental studies, and that’s probably inappropriate.”

Although a lot of these results sound gloomy, they are the first step in addressing the SES gap: acknowledging that SES does make a difference. Not every kid is raised by Ivy League grads that can afford to drive a BMW SUV to a developmental psychology lab, and it’s important to recognize that a family’s wealth and experience play a huge role in a child’s early life.

The next step is to break down the problem. The categorization of SES has many components: education level, employment, income, relative social standing. SES is unlikely to be the direct cause of differences we see in kids; rather, it indicates a host of other factors that vary between families, such as nutrition, stress levels, or the amount of time parents can invest in their children. For instance, in a low-SES family, both parents may need to work long hours, leaving less time to spend playing or talking with their kids. Less language input is correlated with slower language and reading skills at later ages.

A final step is to determine how these issues can be addressed. The good news is that some of the negative outcomes associated with low SES can be reduced by engaging children in active conversation or play. One simple suggestion for parents and educators is to talk with kids more; Fernald found that low-SES children whose parents talk to them more at home do better on language tasks. Of course, this is easier than it sounds—try working a 12-hour day and finding the energy to coo to your child about your day while making dinner. But this kind of research can provide basic tools for educators and policymakers to design programs and interventions that help families raise their children as successfully as possible.

Meanwhile, it’s important for researchers to continue probing the effects of SES. To do this, we need support. Developmental psych, in my experience, anyway, isn’t regarded as a sexy field, and many research groups are underfunded. Without grants to purchase equipment and hire people to develop outreach programs, it’s hard for researchers to explore populations beyond the “convenience sample” of privileged children in the lab. Fernald and Noble are not the only ones seeking out less advantaged children as test subjects, but they are still in the minority.

It’s not yet part of our field’s culture to consider children’s background as a factor, but it should be. “If I reported results about kids in Senegal, people would nail me—they would say, ‘You need to say that’s about children in Senegal,’ ” Fernald said. A similar qualifier should be added to the typical developmental psychology research sample: These are not generic kids; they are kids who have grown up with rich, well-educated parents who have given them a taste for edamame.