Watching a romantic movie can boost progesterone levels more than 10 percent, bringing couples closer together, according to new research at the University of Michigan. A movie like "The Godfather" can alter testosterone and dampen attentive feelings.

U-M psychology researchers tested three different groups. One viewed a romantic scene from "The Bridges of Madison County," a second group watched portions of "The Godfather: Part II" while a third group watched a documentary on the Amazon rainforest. Each had hormonal levels tested immediately before, after and 45 minutes after viewing the 30 minutes of tape.

The rainforest group saw no change in hormonal levels while men and women watching the romantic film saw their need for affiliation and their progesterone levels rise. "Bridges of Madison County" left women’s testosterone levels unchanged, but had a dampening effect on men's testosterone levels.

Men and women reacted totally differently to a "Godfather: Part II" scene where a young Vito Corleone acquired and consolidated power after killing a hated foe. High-testosterone men saw their power motivations and testosterone levels jump as much as 30 percent as their need for affiliation dropped. Meanwhile, high-testosterone women saw their testosterone fall while people with low-testosterone levels who were most comfortable in submissive roles could get downright uncomfortable.

"When you’re watching movies, your hormones are responding, not just your mind," said Oliver Schultheiss, a U-M psychology professor whose work will be detailed in an upcoming issue of the journal Hormones and Behavior. "This also helps explain why certain people like to go to certain types of movies. Affiliation-motivated people like to see romantic flicks. But power-motivated people prefer movies with more action and violence.

"If you want to learn about someone’s personality, look at their video collection or look at what’s on their bookshelves."

So what’s the best date movie for a couple? "They should definitely see the romantic movie and if the guy is high-testosterone, he should just clench his teeth and in exchange, it will help promote the relationship," Schultheiss said.

Higher testosterone levels can help muscle growth, enhance libido and prime people for more assertive, dominant behavior including aggression, he said. Researchers are less clear about the effects of progesterone. Progesterone is known to reduce anxiety. It is also possible that higher levels of progesterone make people more attentive and can prime them for opening up more and being more romantic while decreasing libido.

While both hormones are present in both sexes, women have less testosterone than men.

Note: If no author is given, the source is cited instead.

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