LOS ANGELES — For decades, toy makers believed the industry gospel: Boys want to build things; girls want to play princess.

But now, female CEOs are leading huge corporations, including Yahoo and General Motors, and more women are becoming engineers and mathematicians. Meanwhile, toy companies are realizing that girls want to build bridges and wire circuits.

Parents, too, are demanding playthings that nurture a love of science and math in their daughters, driven in part by nationwide hand-wringing over a lack of interest in science, technology, engineering and math careers.

As a result, construction toys, bolstered by demand from girls, are a bright spot in the $22-billion industry, which has seen other categories stagnate or decline.

Eager to make up for lost time, Mattel in April acquired Mega Brands, known for its construction sets. Giant toy maker Lego has retooled its classic building kits with a splash of purple and themes such as pet salon and beauty shop. Upstart toy companies are designing girl-friendly toys that combine fun with scientific principles.

“It’s baffling that it took this long for toy makers to get on board,” said Jaime Katz, an equity analyst at Morningstar. “If you aren’t catering to the girls’ side, you are leaving half of the market on the table.”

Although building sets were flat last year, the category climbed 22 percent to $2 billion in 2012, up from $1.6 billion in 2011, according to NPD Group. Over those two years, action figures dropped by 2.1 percent and plush toys slid by 5.4 percent.

“This is an untapped opportunity,” said Michael Swartz, research analyst at SunTrust Robinson Humphrey. “The hot product begets copycats.”

Toy makers have challenged traditional gender roles in the past, especially during the feminist movement in the 1960s and 1970s. Then manufacturers started moving away from gender-free toys and sharpened their focus on targeting girls and boys separately. The reason: Toys aimed at one gender were better sellers.

But after years ignoring the space, toy companies have been paying close attention to how girls like to build.

Lego spent four years researching the female market after realizing that girls weren’t demanding its toys as much as boys were, said Michael McNally, senior director of brand relations for Lego Systems.

The Danish company debuted its Friends line in 2012 with girls top of mind: The sets have a bright color palette with lots of purple, and come with more human-like figures.

“It changed the perception that Lego is for boys,” McNally said. “It’s been a gateway for girls.”

Lego’s focus has paid off handsomely. Prior to Friends, only about 10 percent of Lego sets were bought for girls. Within eight months of the line’s launch, that grew to 25 percent, McNally said.