Acceptance of working mothers in the United States has reached a record high, according to a study conducted at San Diego State University.

Moreover, acceptance is likely to continue to increase, even in the case of working mothers with young children, said Jean Twenge, an SDSU professor and one of the paper's lead authors. This is true despite a small backlash in the late 90s, said Twenge, a psychology professor and author of "Generation Me."

The long-term trend is visible among responses given in two publicly available datasets going back to the 1970s. In the 70s, 59 percent of 12 graders said preschool children would be harmed by their mothers working. That percentage dropped to 34 percent in the 1990s and 22 percent in this decade.

The study was published in the journal Psychology of Women Quarterly. The study was led by Kristin Donnelly, who performed the work while she was at SDSU; she is now studying for her doctorate at UC San Diego.

With such a surge in support for mothers in the workplace, policies should be designed to take this into account, the study said.

"Policy makers should recognize that support for working mothers is now a solid majority position in the United States and design programs for working families accordingly," the study stated.

Other studies that measured the actual effects on working mothers and their children have yielded mixed results. A 2011 study from Melinda Sandler Morrill of North Carolina State University, found that children aged 7 to 17 of working mothers have a 200 percent increase in the risk of each of three serious events: overnight hospitalizations, asthma episodes, and injuries or poisonings.

"These effects are robust and do not reflect a non-representative local effect," stated the study, published in the Journal of Health Economics.

Another 2011 study, published in American Sociological Review, found that working mothers spend more time multitasking than do working fathers, and their experience of multitasking is more negative.

However, a 2011 study from the UK indicated there was no significant harm to a child's social or emotional development from having a working mother.

In the new study from SDSU, researchers used two national surveys that included nearly 600,000 people. One was of 12th graders and the other of adults, taken between 1976 and 2013. By studying the answers over time, researchers could plot the evolution of attitudes toward working mothers, Twenge said.

"I'm fortunate in that I didn't have to hand out all those questionnaires," Twenge said. "These are both publicly available datasets. There's still definitely work involved in sorting through these datasets, particularly the high school files. They're all individual years. You have to merge the whole files, clean it up, do all the analyses, and so on. They're both nationally representative."

Because the surveys are done over time, age of the respondents can be removed as a confounding factor, Twenge said. This allowed the researchers to zero in on cultural changes among respondents of the same age across different decades and generations. The high school datasets in particular included many questions about attitudes toward working mothers.