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Ian David Lithgow (born February 3, 1972) is an American actor and therapist. He is best known for his work as Leon in the NBC series 3rd Rock from the Sun (1996–2001). He is the son of actor John Lithgow.

Lithgow was born on February 3, 1972, in New York City, the son of actor John Lithgow.^[1]

He received his undergraduate degree from Harvard University in 1994, majoring in English.^[1] He spent two years doing his M.A. in clinical psychology, specializing in marriage and family therapy, at Antioch University Los Angeles, graduating in 2005. As of 2023^[update] he runs a practice in Lower Manhattan.^[2][3]

Lithgow has acted in film, television, and in regional theatres across the US, including the Goodman Theatre, the American Repertory Theatre, the Williamstown Theatre Festival, the Ahmanson Theatre, and the Pasadena Playhouse. He originated the role of Tony in Boy Gets Girl, which he performed at the Manhattan Theatre Club.^{[citation needed]}

In the 1990s NBC sitcom 3rd Rock from the Sun, Lithgow portrayed the recurring role of Leon, a purportedly dim-witted student who attends the physics class of Dick Solomon (portrayed by Ian's real-life father John Lithgow). Ian portrayed a son of John's character in the 2020 HBO television series Perry Mason.^[4][5]

Lithgow married Rachael around 1993 in Los Angeles, and the couple had two children. Rachel, according to her website "built a career as a professional Jewess, running some of the most important Jewish cultural institutions in the United States, curating major exhibitions, and consulting for institutions around the world." They separated in 2018 but remain good friends.^[6]

• Ian Lithgow at IMDb