January Jones says only her son should know who his father is.

The Mad Men actress - whose ex-boyfriends include Ashton Kutcher and Jason Sudeikis - is remaining tight-lipped about the identity of her 20-month-old son Xander's dad and has taken advice from legendary actor Jack Nicholson in how to approach difficult questions about the subject.

When asked to state who Xander's father was, the blonde beauty told The New York Times newspaper, "That's my son's business. It's not the public's business. Jack Nicholson once told me, 'You should never give your personal life away, otherwise people will pick you apart. They'll never believe in your character.'"

January believes women have a right to keep "secrets" and likes the idea of saving unknown information for a future project, such as a journal. "Women should have lots of secrets. It's our right to have secrets. Otherwise, what would we write in our memoirs?" she added.

Despite her refusal to reveal Xander's second parent, the 35-year-old star was happy to reveal her son's first words and recalled what it was like when she chose to eat her placenta after giving birth in September 2011, describing the act as something natural: "His first word was 'Mama.' His second was 'back,' cause I kept saying, 'Put it back.' [Eating the placenta] was like taking a vitamin blended into a smoothie. I'm a mammal. I nursed. I did all kinds of weird stuff."