By year two, researchers changed their informational materials to better educate the population about their purposes and added a $50 payment.
Science has a dark past with the historically underprivileged — including prisoners, the poor and African-Americans — subject to unethical experimentation in the middle of the last century. In some experiments, they were purposely given diseases or kept from a cure, and ill will from those days lingers in some communities.
Overcoming the mistrust is crucial, researchers said.
"It's extremely important that we test children from all different income levels, because sometimes things come out differently," Lillard said, pointing to research that shows children in poorer households are spoken to less, which impedes their cognitive development.
"We wouldn't know that if people hadn't gone into the homes [of lower income families] and recorded how much language there was."
Feigenson and Halberda, the directors of the Hopkins Laboratory for Child Development, which is located on the university's Homewood campus, hope to set up a sort of satellite operation in East Baltimore, where Hopkins has a $1.8 billion redevelopment effort underway.
They already have a lab set up at the Maryland Science Center at the Inner Harbor, to help educate the public about science and to attract new study participants. They typically recruit people through letters sent to new parents on their behalf by the state Department of Health and Mental Hygiene, and by word of mouth.
"Developmental psychology is one of these like citizen-science fields, where the families that are coming in are contributing to the science," Halberda said. "The science doesn't move forward without the families coming in."