Here's some mildly-persuasive evidence that Todd is Liv's bio-father -- even though I wouldn't seriously argue that, because Liv would know.
1) Attention Deficit Disorder (ADD) has a significant hereditary component
Is ADD Hereditary? (Attention Deficit Disorder)
There are probably multiple causes of AD(H)D. We know that these conditions do clearly run in families. About 30% of individuals with AD(H)D have a close relative with this condition.
2) Liv has ADD
Liv Tyler has revealed she takes time out to meditate to help her deal with attention deficit disorder (ADD) and finds the relaxation technique ''helps with everything''.
He told Myers that he gained complete focus with Ritalin, which led him to quickly churn out hit songs like "I Saw the Light".
Ritalin Ritalin is used to treat attention deficit disorder (ADD), attention deficit hyperactivity disorder (ADHD), and narcolepsy.
And some more that turned up while I was googling around:
Both from an issue of People magazine:
Rock Stars & Centerfolds
1974 Playboy centerfold Bebe Buell has an eight-month romance with Aerosmith frontman Steven Tyler. In July 1977, while in a relationship with rocker Todd Rundgren, Buell (right) gives birth to a daughter she names Liv Rundgren. Rundgren is listed as the father on the birth certificate because Tyler is in the middle of his well-documented drug excesses. "Todd's my spiritual father. I love him," Liv tells PEOPLE in 1992.
WHAT? How the frack does that math work? If the 1974 romance with Tyler started in December 1974 (the most accommodating interpretation) then it ended in July 1975 -- a full two years before Liv's birth!!??
Buell introduces Liv to Steven Tyler at a Rundgren show but not as his daughter. "I didn't know what it was, but I started to see him more, and it would mean so much to me that I would write about it in my diary," she tells PEOPLE in 1992. "I was very confused." When Liv sees Mia (Steven's daughter with his first wife Cyrinda, who is a splitting image of Liv) at an Aerosmith concert in 1988, she realizes Steven is her biological father.
"Spitting image" WTF?
Compare their faces here and here and here. IMO, they don't look remotely alike. Liv has a very long face, like Todd, and her nose doesn't widen at the nostrils, like Todd's. Mia has a round face, like this 1975 photo of Steven.jpg) and this 1978 Steven.
Finally, from Liv's mother's IMDB profile [emphasis added]:
Bebe Buell [born in July 1953] is a former model and Playboy Playmate who dated several rock stars in the 1970s. She began modeling at the age of 17 and moved from her hometown of Portsmouth, Virginia, to New York City. Shortly after moving to New York, she met rock star Todd Rundgren. At the time she was living in a women's home run by nuns. Bebe and Todd began dating (she says she lost her virginity to him), moved in together, and ended up living together for six years. Although she and Todd were in a steady relationship, they both dated other people while they were living together. Her rock star lovers during the 1970s have included Mick Jagger, Iggy Pop, David Bowie, Jimmy Page of Led Zeppelin and Steven Tyler of Aerosmith.
She was born in 1953 and moved to NYC when she was 17, so that was 1970. After a period described as "shortly", she moved in with Todd in a relationship that lasted six years. Liv was born in July 1977, so she was conceived in November 1976. It's not quite guaranteed by that data, but to me it seems very likely that Bebe and Todd were still living when Liv was conceived.
On the hand, apparently Bebe was a rock'n'roll slut (a word I use reluctantly). Who knows? Maybe she was banging Steven (and not David, Iggy, Mick or Jimmy) that November, and when her period skipped, she did the math. "Uh... the father must be... no, maybe, uh,... Steven!"
Unless she had a policy of never fucking Todd when she was fucking other men, though, it would be just a guess. To be an effective policy, it would have to be anticipatory: "I'm going to start fucking Steven Tyler in 30 days, Todd, so to remove any ambiguity about who fathers a possible baby, I'm cutting you off from sex as of today." I doubt that's how it worked with Bebe.
Did they ever do a DNA test? I looked pretty hard. There are a couple Hollywood gossip-type pages where it's alleged that Steven's parentage was verified by DNA test, but not anything that looked like an actual news report... and that would have been highly newsworthy, at least at magazines like People.
TL;DR: Liv has ADD and Todd might have it, suggesting that she got it from him. The story that Liv realized that Steven was her dad because Liv recognized her resemblance to Tyler's daughter Mia seems preposterous: they don't look alike. Liv looks like Todd, IMO. AFAIK, there was no DNA test.
