For decades, toy makers believed the industry gospel: Boys want to build things; girls want to play princess.
But now, female chief executives are leading huge corporations, including Yahoo Inc. and General Motors Corp., 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 STEM careers (short for science, technology, engineering and math).
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 Inc. 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% 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% and plush toys slid by 5.4%.
"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% of Lego sets were bought for girls. Within eight months of the line's launch, that grew to 25%, McNally said.
"We have only begun to scratch the surface," he said.
To win over girls, toy makers say they have to walk a careful line: Avoid pandering by "pinking and shrinking" boy toys but also design a product that is entertaining enough to woo customers.
While boys are often satisfied just by building something cool-looking, girls want more narrative and storytelling in their construction toys, experts say.
The first prototype for Maykah Inc.'s Roominate line was a car, said Bettina Chen, co-founder of the Sunnyvale, Calif., company. Chen and her co-founder, Alice Brooks, later scrapped that idea in favor of a dollhouse.
"The car kit was an educational toy first," Chen said. "We really needed to integrate education and fun in a more seamless way."
The Roominate dollhouse is anything but traditional. Circuits are wired that power lights and a working fan. Later sets enable kids to build miniature spinning windmills and elevators that go up and down.
The growing popularity of construction toys for girls also reflects a shift in family dynamics. Parents are increasingly more open to toys that cross traditional gender stereotypes, and dads are taking a bigger share of child-care duties.
"Parents are telling kids it's OK to be different," said analyst Swartz, who pointed to Hasbro's introducing an Easy Bake Oven with a color scheme more appealing to boys.
"Little boys are not berated by their fathers for playing with those kinds of toys," he said. "Parents are telling their girls that you don't have to be constrained to a life at home cooking and cleaning."
Toys R Us is seeing more fathers buy building playthings for their daughters, said Richard Barry, the company's chief merchandising officer.
"There is absolutely a pattern of dads buying and building Legos with their daughters," he said. "It introduces a play pattern that maybe girls would like."
Dulce Aguilar said she would consider buying a girl-oriented construction set for her goddaughter, who is not interested in "typical princess gifts."
The 39-year-old elementary school teacher said she's noticed that building blocks are a hit with both her male and female students, and she's glad more toys are designed with girls in mind.
"It's a great opportunity for girls to learn skills in terms of building and how to support a structure," the Montecito Heights resident said.
"I like the gender-neutral stuff, but some little girls will gravitate to pinks and purples rather than reds and blues."
Some of the biggest innovators have been women with science backgrounds who are tired of seeing fashion dolls and necklace-making kits dominate the toy store.
Chen and Brooks of Roominate met as engineering grad students at Stanford University. Another Stanford grad with an engineering degree, Debbie Sterling, founded GoldieBlox, one of the most talked-about toy start-ups this holiday season.
Sterling said she got into construction toys after seeing the lack of female engineering majors in school. Her goal: to create toys that nurture an early interest in science and math in girls, the way that building sets have done for boys.
"Girls are just inundated with toys and characters that don't motivate or encourage anything beyond showing them it's important to be beautiful," she said. "Most in the toy industry told me the idea would never go mainstream and in order to appeal to girls, it had to be sparkly with ponies."
After raising money on Kickstarter, the Oakland company rolled out a pastel-colored construction set that came with a book featuring the adventures of Goldie, a girl inventor.
The sets ask girls to build belt drives and work with levers and hinges. One teaches kids how to make movies by crafting a rudimentary animation device.
This year, GoldieBlox even aired a Super Bowl commercial after winning a competition sponsored by Intuit. The company has enjoyed rapid growth, and revenue is set to triple from last year, Sterling said. The private company doesn't disclose sales. In 2013, it sold out of every toy during the holidays.
GoldieBlox has been working to add more diverse characters. One of Goldie's new buddies, an African American girl named Ruby, is a programmer and social butterfly.
"She really shatters the perception that all coders are guys in hoodies who don't shower and have no friends," Sterling said.
Dave Plenn, owner of independent toy store the Dinosaur Farm in South Pasadena, said he was initially "a little skeptical" that girls wanted pastel-colored construction sets.
"I thought it was kind of missing the point," he said. "Girls who like building things, it doesn't matter if it's pink or not."
Turns out, girls really do like construction sets designed for them, Plenn said. The store has already sold out of three kinds of GoldieBlox sets; Plenn is ordering more for the holidays.
"I was wrong," he said. "It lures them in, and they start building things."