Researchers at the Kinsey Institute for Research in Sex, Gender, and Reproduction at Indiana University questioned focus groups involving 50 men divided into three groups based on their age -- 18 to 24 years, 25 to 45 years and 46 and older.

"We use all kinds of methods to measure men's sexual responses; in addition, we use questionnaires and surveys to ask about sexual behaviors," Erick Janssen said in a statement. "It's less common to sit down with men and ask them to talk about their experiences."

The focus groups revealed:

-- Most of the men said they can experience erections without feeling aroused or interested.

-- Many men found it difficult to distinguish between sexual desire and sexual arousal.

-- Sexually experienced women were considered more threatening by younger men, who had concerns about "measuring up," but such women were considered more arousing for older men.

The findings are published in the journal Archives of Sexual Behavior.