U.S. smartphone penetration is nearly at the halfway mark as of January, according to the latest Nielsen data. The 48% of American adults with smartphones is up from 44% in the third quarter of 2011. It likely reflects a bump from the record 37 million iPhones that Apple sold worldwide in the fourth quarter.

Nielsen tends to be the most aggressive among research firms when it comes to estimating how many Americans have smartphones. Most put the level closer to 40%. comScore, for instance, estimated smartphone penetration as of December at 42%. The Nielsen findings are based on a survey of 20,000 mobile users.

Younger adults are more likely to have smartphones. The 24-34 age group had the highest proportion of smartphone owners at two-thirds. That was followed closely by people ages 18-24, at 62%, and people 35-44, at 58%.

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