Four in ten adults in the United States have watched a video on YouTube. Of those that visit the video clip web site frequently, one in three said they are watching less television as a result. Three quarters of them said they would visit the site less often if adverts were shown before each video.

These were among the findings of a recent Harris Interactive poll of a representative sample over two thousand adults in the United States. While two thirds of those that had watched a video on YouTube had only visited the site a few times, 14% of them described themselves as frequent viewers.

Of the frequent YouTube users, two thirds reported that they were sacrificing other activities, with 36% doing so at the expense of visiting other web sites and 32% were watching YouTube rather than television. YouTube also appears to cut into other activities, including email and the use of other social networking sites, work or homework, playing video games, watching DVDs and even spending time with family and friends.

YouTube is perhaps predictably more popular among young males, a group that advertisers already find hard to reach through television. Over three quarters of 18 to 24 year old American men said they have watched a video on YouTube, with over four in ten visiting the site frequently.

Aongus Burke, senior research manager at Harris Interactive, said that YouTube has emerged as a major force in, and problem for, the traditional entertainment industry. “Not only is YouTube using a lot of their own content to steal the eyeballs they want the most, the site has provided a launching pad to wholly new forms of user-generated video entertainment that are gaining popularity quickly.”

However, it seems that users of YouTube may be resistant to any plans to introduce advertising in association with clips. When asked if the inclusion of short commercials before every clip would change how often they will visit YouTube, nearly three-quarters of adults who frequently visit the site say they would visit it less often as a result.

“We have seen in previous data,” said Burke, “that consumers as a rule are not averse to watching commercials online in order to catch an episode of a TV show they would otherwise miss. Yet those who are accustomed to finding and watching everything for free at YouTube may have developed a very different set of expectations for the site.”

Nearly three quarters of those in the total sample of 2,300 adults reported watching videos online, rising to 87% for those aged 25 to 29. YouTube was the most popular destination, particularly among those under 25. Older groups were as likely to have viewed video on a television network site, particularly those over 50, although 13% of those over 65 still reported having seen a video on YouTube. 47% of men said they had seen a video on YouTube compared to 36% of women, but over half of all those aged 25 to 34 had visited the site.

The headline results of this survey suggest a significant threat to broadcast television viewing. It should be noted, however, that fewer than 10% of the 42% of adults that had seen a video on YouTube watched more than an hour of week on the site. Just under 6% of all those surveyed reported watching less television as a result of YouTube.

On average, Americans still spend well over an entire day a week watching television. Nevertheless, the impact of YouTube is impressive.