Special offers on many of the popular smartphones in the fourth quarter drove penetration to nearly a third in the U.S., according to The NPD Group, a wireless market research.

The report found that smartphones are now in the hands of about 31 percent of the U.S. population, growing from only 23 percent in the year-ago period. Of those phones purchased in the fourth-quarter, nearly two-thirds cost $150 or less, and many more were bought for even less. “Although we are seeing more expensive models among the best-selling handsets, carriers are now offering some popular smartphones for less than $100,” said Ross Rubin, NPD’s executive director.