Law of Inverse Paternity

The odds that a given character is the biological father of someone whose paternity is unknown is directly inverse to how badly the other parties directly concerned want him to be the biological father.

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.

So, Alice is pregnant and caught in the horns of her Triang Relations - 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 Charlie, the hypotenuse—her worst nightmare.

Examples of Law of Inverse Paternity include:

Anime and Manga

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.

Film

This came into play in Snow Dogs (starring Cuba Gooding, Jr.). Our protagonist goes to Alaska to find his real father. He develops mutual hate 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.

Live Action TV

Casualty: played straight with Selena realizing 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 was 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.
Ugly Betty: A variation - Henry finds out that he was father of Charlie's baby.
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 Friends when Chandler and Monica find out their adopted children could have been fathered either by a clever student, or by a man who killed his father with a shovel. Chandler seems to think this trope will come into play ("Honey, it's us! Of course it's the shovel killer!") but it turns out he's wrong. Turns out that what the girl and the killer did couldn't cause pregnancy. ("Was it the thing we rarely do, or the thing we never do?" "The thing we never do." "All right, Shovely Joe!")
- Also averted with Rachel's unplanned pregnancy when the dad is Ross, her one true love, and not the random way too young dude she just broke up with.
Done three times with the same pregnancy on Neighbours. Bear with me: 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.
In one episode of John Doe the boy and the title character want him to be the father, but he isn't.
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.
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.
Subverted on Community, when Shirley's baby turns out to be her ex-husband's rather than her one-night-stand Chang's.
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.

Theatre

Older Than Feudalism: This happened in Oedipus the King after Oedipus learned that he was adopted.

Web Comics

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. In a more recent strip he admits that in retrospect, he wishes Rory had been his son.

Western Animation

Subverted in Futurama: 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.