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 realising 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 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 realising 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 colour 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.
"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 colour 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."
Dave Plenn, owner of an independent toy store, said he was initially "a little skeptical" that girls wanted pastel-coloured 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."
Tribune News Agency