Despite TV's widespread reach, the attention of its US audience is something that's increasingly fractured across several devices.

That development is reflected in the growing trend of second-screening. eMarketer estimates that 185.8 million adults in the US will regularly use the internet on a second-screen device while watching TV this year, an increase of 4.5% from last year's figure.

New research from YuMe and Nielsen found that there are some generational differences in the device most used for second-screening.

In the firms' January survey of US internet users, the same percentage of millennials—defined as ages 20 to 36—used smartphones to second-screen as used PCs, at 68%.

Among both Gen Xers and baby boomers, the use of smartphones as a second screen was significantly less prevalent. Only 52% of Gen Xers and 39% of baby boomers fired up their smartphones while watching TV. Instead, desktop/laptop remained the device of choice for second-screening among these groups.

Tablets were used by about the same proportion of respondents from all three cohorts, at roughly one-quarter.

Despite these figures, eMarketer predicts that smartphones will be much more widely used as a second-screen device than desktops/laptops in the the coming year.

We project US adults using smartphone internet and TV simultaneously will number 176.1 million in 2018. Meanwhile, the number of desktop/laptop internet and TV users will reach just 110.0 million.

In addition, data from PwC reveals that US internet users are engaged in a wide variety of behaviors on their second screens. The company's October 2017 survey found that looking up actors or players in a show or sports game was the most popular activity, but people were also communicating with friends and using social media in ways completely unrelated to TV programming.