Or, to put it in plainer terms: If it is strongly hoped that a given character is the biological father of a character of unknown paternity, he probably won't be. If it is strongly hoped that a given character isn't the biological father of a character of unknown paternity, he even more probably will be.
A Sub-trope of Who's Your Daddy?, although it may also be coupled with Law of Inverse Fertility if it comes into play early enough. Owes its existence to the Rule of Drama, much like many other tropes.
So, Alice is pregnant and caught in the horns of a Love Triangle—she doesn't know who the father is. So, after sufficient tension has been built up, she discovers that her baby's dad is not Bob (her and the fans' one true love) but Charles, the hypotenuse—her worst nightmare. The law applies to the men too: Bob will hope he's the father before anyone knows for sure and be disappointed when the truth is known, while Charles will have much stronger misgivings than Bob (often treated sympathetically) about taking on the responsibilities of fatherhood.
- In Nana, after starting a relationship with Nobu, Hachi sleeps with Takumi then discovers she is pregnant. Since she and Nobu practiced safe sex, Takumi must be the father, so she ends up with him.
- In Snow Dogs (starring Cuba Gooding Jr.), the protagonist goes to Alaska to find his real father. He develops mutual hatred at first sight with an old and mean dogsledder. Unfortunately, circumstantial evidence suggests that this fella is his father, for various reasons, including a shortage of plausible choices and a mutual liking for blue cheese. And it turns out to be true.
- A favorite of soaps, period, playing out exactly as described at the top of the page.
- Surprisingly averted on All My Children. When a grieving Maria had a one-night stand with an equally grief-stricken (over separate issues) Dimitri and later learned she was pregnant, she was relieved to learn that he was not the father of her baby, husband Edmund was.
- And on The Bold and the Beautiful, when Amber trysted with both fiance Rick and buddy Raymond. Amber was quite relieved when the baby was very obviously Rick's—white, as opposed to black, like Raymond.
- And on General Hospital, when Brenda slept with husband Jax and ex-lover Sonny, only to not get pregnant at all.
- Subverted in Passions, when Theresa's son, Little Ethan—who, up until then, was thought to have been fathered by Jerkass Julian Crane during a drunken one-night stand, in accordance with The Law of Inverse Fertility—actually turned out to be the son of his namesake, Ethan. There was much rejoicing, as Ethan & Theresa were Passions' One True Pairing.
- Selena realizes that Nathan was the biological father of her baby, not Harry.
- In its Spin-Off, Holby City, the Williams family history has become particularly contorted by the result of a paternity test; it is revealed that Chrissie was conceived by the rape of her mother by her presumed father's father, making her presumed father her actual half-brother.
- Subverted on Community, when Shirley's baby turns out to be her ex-husband's rather than her one-night-stand Chang's.
- Friends: Subverted when Chandler and Monica find out their adopted children could have been fathered either by a smart scholarship student, or by a man who killed his father with a shovel. Chandler thinks this trope will come into play, given they're already victims of the Law of Inverse Fertility. However, it's Subverted and Chandler is wrong. Turns out that the sex act the girl and the killer did couldn't cause pregnancy.
- Chandler: Honey, it's us! Of course it's the shovel killer!
- In one episode of John Doe the boy and the title character want him to be the father, but he isn't.
- Played With on Living Single—Max gets pregnant by a sperm donor, and later discovers that, by Contrived Coincidence, the donor was Kyle, with whom she had a major Slap-Slap-Kiss, Enemies With Benefits-type relationship before he left the country. The finale has him come back, find out and agree to raise the child with her.
- On Lost, Sun is worried that the father of her child is the man she had an affair with, rather than her husband Jin's. Juliet helps her confirm the date of conception. It's Jin's.
- The talk show Maury lives on this trope. The most frequent episode format involves a woman seeking a paternity test for her child, often with a man (or multiple men) she hates so that she can spitefully hit him up for child support. The more tearjerking variant involves married couples where the husband is heartbroken to learn that his beloved child isn't biologically his due to his wife's affair.
- On My Name Is Earl, Joy has an affair with Darnell while she's still married to Earl, after the stress of taking care of a colicky Dodge took a toll on the marriage. Several months into the affair, she finds out she's pregnant and excitedly shows Earl the positive test. Then she has an Oh, Crap! moment as she realizes that there's a good chance the baby isn't Earl's... and that there's a good chance of delivering a Chocolate Baby. She even asks the doctor who's giving her an ultrasound to see if he can tell the baby's race. In keeping with this trope, she gives birth to a black baby... and Earl almost leaves her. It turns out that Earl Jr. might not be Darnell's, either, based on a Daddy DNA Test, but that plot point was unresolved thanks to the 2007 TV Strikes.
- Done three times with the same pregnancy on Neighbours. Bear with us: first Sky was with Dylan, but cheated on him with her art teacher, breaking them up. A few weeks later, Sky has a one night stand with Dylan's brother Stingray, who is also her best friend. Shortly afterwards, she discovers she is pregnant. When Stingray asks if it could be his baby, she says that she wishes he was, but the dates don't add up. As soon as Dylan has accepted he is the father and become excited, a scan shows that the baby is younger than originally thought, making Stingray the father. This destroys the relationships of Dylan and Sky, Stingray and Rachel, and Dylan and Stingray. Then when the baby is born, a DNA test shows Dylan is the father after all—the baby's size is attributed to Dylan's exposure to toxic waste a year earlier, which later causes Kerry to develop cancer—but Sky's relationship with Dylan is over permanently, and she ends up with Stingray.
- Poldark: It would be better for pretty much every character if Elizabeth's baby in Series 3 were her new husband George's. Naturally, it's Ross's, and the result of a one-night affair.
- Brazilian Soap Opera Por Amor: The main "villainess" raised her two sons very differently because she believed that her favorite one was the one she bore with the man she was and still is truly in love with... who, incidentally, isn't her husband of 30 years. All the men involved learn about it, DNA tests ensue, and it's revealed that the biological son of her true love was The Unfavorite. Cue her being abandoned by her family and her ex-lover, and a Villainous BSoD.
- Subverted in the David Tennant four-part drama Single Father. The main character Dave lives with his three kids and his partner's daughter from an earlier (and extremely brief) relationship. Suspicions arise that the other three kids were also fathered by the same man as the daughter, so Dave goes for a paternity test and it's obvious that the writers are setting this trope up... But no, turns out they're his after all.
- On Supernatural, Dean has his girlfriend Lisa and her son Ben has a lot in common with Dean. Ben is about the right age to be conceived from a previous fling between Dean and Lisa, and he wishes he were the father, but Lisa makes it clear that he isn't. It turns out it was some guy in the back bar of a biker joint with a "leather jacket, couple of scars, (and) no mailing address". She admits she has a type.
- Ugly Betty:
- A variation—Henry finds out that he was the father of Charlie's baby.
- Done again when Hilda ends up pregnant with Bobby's child, who her family disapprove of, instead of Archie's, who is she is dating.
- The Remarried Empress: It's revealed that it was Sovieshu who was left infertile by a childhood incident and not Navier as he initially believed. Which means he isn't the father of Rashta's baby as he was hoping to secure a needed heir, rending the whole reason he divorced Navier to marry Rashta All for Nothing.
- Played with in Something*Positive: Davan doesn't want to be Rory's biological father, but while he's willing to step up, the other (unnamed, unseen) guy isn't. Even when Davan is cleared by a DNA test, he becomes Rory's Parental Substitute anyway. He later tells Vanessa that in retrospect, he wishes he had been Rory's father; not long after that, Rory asks if he can call him "Dad" from now on.
- Subverted. Kif's species can get pregnant just by coming in contact with another's DNA; the person whose DNA he assimilated (Leela) wasn't his first choice, but at least it wasn't Zapp Brannigan. However the parent, or smizmar, is considered to be the one who got them turned on into DNA assimilation mode in the first place, who in this case happens to be Amy.
- Used in the movie Bender's Game. The father of Mom's son Igner turns out to be none other than Professor Farnsworth, one of the smartest men in the universe, who is constantly complaining about how stupid Igner is.