Once upon a time, a typical gift for a child was a set of blocks. Plain old blocks with no batteries or screens, no electronic voice asking to be friends, no game of Angry Birds somehow embedded in their cubic walls.

No longer.

As anyone who braved toy stores this past holiday season knows, the bulk of gear for children these days is far more technologically decked out, with everything from flashing lights to 3-D computer screens to disembodied voices. And this, say child development experts, is turning into a massive problem.

High-stimuli toys, even many of those advertised as "educational" or "interactive," actually serve to diminish children's creativity, many experts say. Instead of using their minds to imagine how to use a toy – how to build a castle with blocks, say – they simply push a button or watch a flashing light. The toy is doing the work, which is the reverse of what researchers say is ideal.

"The best toy is 90 percent child and 10 percent toy," says Susan Linn, a Harvard University psychiatry instructor and cofounder of the Campaign for a Commercial-Free Childhood. "The [perfect] toy's meaning and its use changes at the child's behest."

At the same time, a large percentage of children's toys are based on media characters – Transformers, for instance, were top sellers this past holiday season. The problem with this, says Diane Levin, an education professor at Wheelock College in Boston, is that when a child plays with a toy that already has a character description, the play tends to be limited; the child doesn't invent the figure's personality or actions because those characteristics are already determined.

"Play material is very important," Ms. Levin says. "When they have something that is just something they saw on TV, they will use it the same way. They will imitate."

And of course, there is the issue of screen time. According to the Kaiser Family Foundation, 8-to-18-year-olds now spend 7.5 hours a day in front of one or more screens. This, according to the American Academy of Pediatrics, is too much. It recommends no screen time for children 2 years old and younger, and no more than two hours a day for older ones.

But some top-selling toys this past season – including infant toys – were screen-based. (Teachers Resisting Unhealthy Children's Entertainment named a tablet computer for babies the worst toy of the year; a similar device was in the Toys "R" Us Top 15 Christmas gifts for 2011.)

Many of these screen toys advertise themselves as educational, tapping into parents' desire to help children get ahead in a technologically focused world. In a December Monitor TIPP poll, for instance, two- thirds of Americans agreed with the statement "the earlier a child can use technology, the better off he or she will be." Yet numerous studies have found no educational benefit – and possibly some harm – in early screen time.

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It comes down, child advocates say, to money.

"One of the reasons that creative play has been diminishing in the United States is that it's not lucrative," Dr. Linn says. "Companies make less money when children play creatively. Children who play creatively need less stuff, and they can use the same thing over and over again – mud, water, blocks, dolls that don't do anything."