If you have purchased a television this year, you may not realize that it has a built-in feature — the much-ballyhooed “v-chip” — that allows you to block objectionable shows.

But according to a University of Pennsylvania study, even if you know you have such a television, you’re not likely to use the v-chip. And if you have Internet access at home, chances are you’re not using blocking software, the same study reports.

All new televisions 13 inches and larger sold since Jan. 1 have come equipped with v-chips, per a 1996 law. But the study shows that only about half of those who own a v-chip equipped TV actually use the technology.

The statistics are worse for filtering software that blocks out objectionable Web sites: Only about one-third of families with Internet access use such software. Part of the problem is ignorance; not many people realize that the TV ratings shown at the start of each show can be used to block programming they don’t want their children to see.

Another part of the problem is the difficulty of using the v-chip and blocking software. Depending on the type of TV, a v-chip can be fairly easy and flexible to program, or almost impossibly convoluted. The same holds true of software that blocks Web sites.

But the larger problem is content. As the study noted, most parents don’t understand how the TV ratings system works. For example, seven out of 10 parents surveyed thought “Oprah” and “Who Wants to Be a Millionaire?” were labeled as educational programming — even though the label is actually applied to programs such as “Sesame Street” and “Nova.”

In the end, what parents want their children to see on TV and the Internet is a matter of parental influence. Technology can help parents enforce such choices, but it can’t replace their guidance, judgment and vigilance. That seven out of 10 parents saw no distinction between Regis Philbin and Big Bird suggests this is not a hardware problem.