One of the chief character relationships in American Dad! is the simultaneously loving and confrontational connection between Stan Smith and his eldest child, Hayley. Their conflicting personalities have always been a major element of the show, with the father/daughter duo having many episodes focused on their bond. But one episode revealed that this relationship might not even be based in blood. This is how American Dad! suggested that Hayley wasn't actually biologically related to Stan.

"The Kidney Stays in the Picture" was a part of the show's seventh season. Hayley and Francine spent a night out drinking with Francine's old friend, Kelly. However, while Francine could keep up with Kelly's impressive amount of alcohol consumption, it proved too much for Hayley, resulting in the young woman suffering from severe kidney damage. While Francine was unable to donate a kidney, Stan quickly volunteered to do so, only for Francine to reluctantly and sadly admit that there was a chance he wasn't a match because he might not be her biological father. Francine explained to Stan how shortly before their wedding, Francine and Kelly went out to party and ended up having a one-night-stand with a man she never formally met -- leaving Francine potentially pregnant with his child.

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Stan and Francine utilized a CIA time machine to travel to the night in question to learn the identity of the mysterious man so they could take a kidney from him if they needed to -- and for Stan to try and get some payback for all the time, energy and money he invested in raising Hayley. Stan even almost prevented the past incarnation of Francine from sleeping with the man, potentially invalidating Hayley's very existence. But after coming to terms with the fact that he loved Hayley as his daughter even if she wasn't directly related to him, Stan ensured the past happened like it was meant to. Stan chose to donate his kidney (and had Roger steal a kidney from the other man for security), and made everyone promise to not reveal to him which kidney was used in the procedure, leaving the question of Hayley's birth father unanswered.

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It's still an interesting question that remains unanswered in the series. In the episode itself, Stan pointed out how this could be the reason why he and Hayley had so many personality differences, as they might not be formally related. Only complicating matters, the man who could be her birth father -- Joel Larson -- also wore a distinct green headband similar to Hayley's. However, there are other elements of Haley that bear a stronger similarity to Stan -- including their deep-seated sense of stubbornness, jet black hair and physical prowess (as revealed by Stan's CIA training and Hayley's skill-set from episodes like Season 3's "Haylias").

If nothing else, Hayley is truly Stan's daughter in spirit, sharing a wealth of love with the man despite their differing views on the world. It was an emotional breakthrough for Stan -- who in prior episodes had been quick to condemn Hayley for clashing with him -- and spoke to just how much he loves his daughter, regardless of her true blood relations.

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