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Messner was born on April 11, 1970 in Syracuse, New York. He lived in Europe for 13 years while his father served in the United States Air Force, mainly in Germany and England. During this time, he developed love for sports while playing on teams at military bases. He has one brother and one sister. According to one of his tweets, he was adopted.
Messner has also starred in three movies with Bruce Willis, with whom he is very close friends. The first movie they made together was Tears of the Sun (2003), second was The Whole Ten Yards (2004) and third being Hostage (2005). In one of the interviews for the movie Anacondas: The Hunt for the Blood Orchid he showed the crook of his left elbow, where he has a tattoo of Italian actress Monica Bellucci, whom he met while making Tears of the Sun with Bruce Willis. In that same interview he proudly announced that Bellucci in fact was present when the tattoo was made.
Messner has dated Selma Blair. Currently, he has been in a long term relationship with Kathryn Morris, whom he met while shooting Cold Case. The relationship was long suspected but not confirmed, until the couple opened up about their life together on Brett Gursky's podcast "On The List" on the HAHAJK network in April 2013. The couple welcomed their twin boys Jameson West Messner and Rocco McQueen Messner to the world on August 21, 2013