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Lets say Alice gets pregnant and claims she slept with Bob and he is the father. She wants a paternity test to prove it. The problem is that Bob has an identical brother, Billy, who has the exact same genetics. A paternity test can prove one of them is the father, but not which one she slept with.

Presume that Alice had neither a long standing romantic relationship with Bob nor anyone that was a witness to the sexual act, thus making the question of who she slept with difficult to prove. Can Alice still get child support, or will she be denied because she can not definitively prove which man is the child's biological father? Can she even get a paternity test given that it would not be definitive proof which man was the father?

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    I would imagine that it would be a matter of establishing which brother can provide an alibi for the night of the... um... affair. Presuming that the date of the date is know, it's a simple matter of looking for evidence of Bob or Billy in the area.
    – hszmv
    Commented Nov 28, 2022 at 18:32
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    Incidentally, about 10% of the time, identical twins have a detectable genetic difference, when mutations occur after the embryo splits.
    – user6726
    Commented Nov 28, 2022 at 23:18
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    @hszmv You need an alibi for about a week, two would be better. See my overly long comments on another answer. Commented Nov 29, 2022 at 15:49
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    @NeilMeyer this is not husband and wife, as I understand, but from something like a one-time affair. And even if Alice can distinguish them, the question is about proving it. Commented Nov 30, 2022 at 0:26
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    @NeilMeyer Even if you can tell twins apart, it doesn't tell about their genetics, for example if one is a fat smoker and the other skinny athlete. Alice could want to scam them to get the richer twin to be the father.
    – Boat
    Commented Nov 30, 2022 at 13:30

4 Answers 4

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Can Alice still get child support, or will she be denied because she can not definitively prove which man is the child's biological father?

The legal standard is a preponderance of the evidence (i.e. more likely than not) and there is plenty of evidence that can be offered in addition to DNA evidence, such as testimony under oath from people in a position to know who was having sex with whom at the relevant times.

Contrary to a common misconception, testimony under oath is still solid evidence that can support a verdict on appeal.

Alice had neither a long standing romantic relationship with Bob nor anyone that was a witness to the sexual act, thus making the question of who she slept with difficult to prove.

It isn't that hard to prove.

Q to Alice's physician: Based upon an ultrasound, when did Alice conceive?

A: April 5-8, 2021.

Q to DNA expert: Based upon the DNA test, who could the father be?

A: Billy or Bob.

Q to Alice: Did you have sex with Billy between April 5-8, 2021?

A: No.

Q to Alice: Did you have sex with Bob between April 5-8, 2021?

A: Yes.

Q to Billy: Did you have sex with Alice between April 5-8, 2021?

A: No.

Q to Billy: Why not?

A: I was at the Shuffleboard World Cup in Tibet, I have time stamped pictures.

Q to Bob: Did you have sex with Alice between April 5-8, 2021?

A: -- if Yes, judge says he believes Bob and Alice and the case is over. -- if No, the judge decides who among Bob, Billy, and Alice the judge believes based upon other evidence.

Ultimately, the judge has to rule between the two based upon non-genetic evidence and resolve credibility disputes just as in any other case that doesn't involve DNA evidence (which is the vast majority of cases).

Also, the edge cases are few are far between. Identical twins are rare to start with, and few women have sex with more than one identical twin in the several day period when she could have conceived or didn't know which twin she had sex with. It has happened at least once in history (post-DNA testing), but you can probably count the number of times that it has ever happened on one hand.

For example, presumptions from cohabitation, marriage, and claims of paternity often resolve paternity disputes without DNA evidence.

Further, to the extent that there is good faith uncertainty (perhaps everyone agrees that the mother has sex with both twins on the only possible day of conception and nobody really knows), the downsides to a mistake in the larger cosmic sense of the overall paternity law system are minimal, as identical twins very rarely become deeply alienated from each other and instead tend to be close and intensely cooperative once they discover each other, and tend to be similar to each other in almost every respect depriving the child of little if the court gets it wrong. Realistically, identical twins are particularly likely to settle out of court so the judge doesn't have to decide.

In one of the only two actual cases I could locate that went to trial (in Brazil), both twins were ordered to pay child support because the evidence showed that they actively conspired with each other to confound the mother and the court regarding who the father was, and conspiracies can support joint and several liability.

The other case reported in a news story had convincing circumstantial evidence supporting one identical twin over the other that probably establish a presumption of paternity for one twin and not the other.

One of the twins, who cannot be named for legal reasons, went to court last summer in the hope of forcing the mother to grant him access to the child. Although his name is not on the birth certificate, he claims he is the only father the boy has known, cared for him every other weekend, provided financial support and was even known to him as 'papa'.

But then the man's relationship with his girlfriend broke down and the visits halted. When he began legal proceedings to prove his paternity, the mother made her claim that she had been sleeping with his twin at around the same time.

The twins have said they knew they were both having sex with the woman, but argue that only one had sex during the period of conception. Both refused to undergo a DNA test: the complainant refused to pay the £335 charge while his brother, who has since married and fathered children, does not consider himself involved in the dispute.

Now, however, Judge Jolin has asked the complainant to take a DNA test by 1 December to ensure he can claim even possible paternity, while his brother may also be tested.

(The second case is in Quebec and the cost of the test in pounds is apparently a currency conversion value.)

(It is possible in principle to distinguish even identical twins from each other with high coverage whole genome tests that would reveal a few random mutations in each twin out of billions of possible mutations, but it is currently prohibitively expensive to do so.)

Can she even get a paternity test given that it would not be definitive proof which man was the father?

Yes. This rules out all 4 billion men in the world minus two of them. It has great probative value, narrowing the list of possible fathers down to two.

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    "Q to Billy: Why not?" - I guess both the questioner and Billy know what this question is supposed to mean, but to me it is quite a funny thing to ask someone under oath.
    – kaya3
    Commented Nov 29, 2022 at 7:17
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    @kaya3 - had to laugh at that too! Also, "This rules out 4 billion men"... Alice sure gets around!
    – komodosp
    Commented Nov 29, 2022 at 8:35
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    "only possible day of conception" just doesn't exist. I mean, it wouldn't surprise me if some court used it that way, but that's because courts are notoriously bad at science. For the curious: sperm can be viable for several days (~3?ish) and an egg can be fertilized up to ~a day after ovulation, so even if the moment of ovulation is known precisely you still have a ~4 day window around that for intercourse. Commented Nov 29, 2022 at 15:38
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    ... But even knowing the date of ovulation is hard. Even if you're measuring your hormones every day you can have multiple hormone spikes. A more normal case, you're just guessing based on "previous period"--but while the length of time between ovulation and menstruation is usually pretty consistent for individuals, the time between menstruation and the next ovulation can be all over the place. Doctors can also look at how developed the baby is at birth and work backwards, but that's still an estimate. You need an alibi for at least a week, preferably two. Commented Nov 29, 2022 at 15:43
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    And humans actually have relatively consistent gestation. If you're a giraffe potential-dad you need an alibi for about 3-4 months. Commented Nov 29, 2022 at 15:46
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In British Columbia, Part III of the Family Law Act says that the child's parents are the birth mother and the child's biological father.

Outside of the context of assisted reproduction, there are a series of presumptions that apply unless the contrary is proved. A "male person" is presumed to be a child's biological father in any of the following circumstances:

  • he was married to the child's birth mother on the day of the child's birth
  • he was married to the child's birth mother and, within 300 days before the child's birth, the marriage was ended by his death, a divorce, or voiding
  • he married the child's birth mother after the child's birth and acknowledges he is the father
  • he was living with the child's birth mother in a marriage-like relationship within 300 days before, or on the day of, the child's birth
  • he has acknowledged that he is the child's father by having signed a statement or agreement

If by operation of the presumptions, more than one person may be presumed to be the biological father, then no presumption applies.

If parentage can be resolved by one of these presumptions (and based on the facts in the question, it isn't clear that any of the presumptions applies), that answers the question, unless someone attempts to prove that the presumption is incorrect.

If someone wants to disprove the presumption, or to establish parentage when no presumption applies, they can apply for an order declaring parentage. When hearing such an application, the judge will consider all relevant evidence, including testimony of parties and witnesses, physical evidence, genetic testing, expert opinion, etc. The burden is on the party seeking the order, on a balance of probabilities.

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    "male person"; I like that the law writer remembered that even though two female people can get married, it's still not reasonable to presume either of them got the other pregnant.
    – Clumsy cat
    Commented Nov 29, 2022 at 8:29
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It may depend on the location of the dispute, but my understanding is that most states impose a preponderance-of-the-evidence standard in paternity actions. Alice therefore does not need to "definitively prove" who the father is; she merely needs to prove that it is more likely than not that Bob is the father.

Armed with the paternity test, Alice has already made it a 50/50 proposition that Bob is the father, but that is not sufficient to establish a preponderance of evidence. She will therefore need to provide some additional evidence identifying Bob. She might attempt to make this showing with a complicated forensic analysis that tracks his phone GPS data to her apartment nine months before the child was born, but it also might be as simple as saying that she can tell the difference between the two and the she knows it was Bob.

If it's enough for the judge or jury, then she'll likely be eligible for child support.

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    FWIW, I am not aware of any U.S. state or territory in which paternity cases are heard by juries, and there are definitely no non-U.S. jurisdictions that do so.
    – ohwilleke
    Commented Nov 28, 2022 at 19:02
  • Sounds correct. I've never been involved in such a proceeding and definitely don't know the answer.
    – bdb484
    Commented Nov 28, 2022 at 19:11
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    Actually, it looks like at least Florida calls for jury trials: B.J.Y. v. M.A., 617 So. 2d 1061 (1993) ("[T]he Florida Constitution preserves the right to a jury trial in paternity proceedings, and we declare unconstitutional the portion of section 742.031, Florida Statutes, that requires paternity proceedings to be tried only by the judge.")
    – bdb484
    Commented Nov 28, 2022 at 19:13
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    NC too. Bell v. Martin, 299 N.C.715, 264 S.E.2nd 101 (1980). One of the issues in this case was the defendant’s argument that because paternity must be proven beyond a reasonable doubt pursuant to N.C. Gen.Stat. §49-14(b), that the proceeding to establish paternity was quasi-criminal in nature and therefore required a trial by jury. The Supreme Court disagreed and held that a paternity action under N.C.Gen.Stat. §49-14 was a civil action and that the defendant had a right to a trial by jury but that the right could be waived by failure to make a demand for a jury trial.
    – ohwilleke
    Commented Nov 28, 2022 at 20:02
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    NE too. Who knew? nebraskalegislature.gov/laws/statutes.php?statute=43-1412
    – ohwilleke
    Commented Nov 28, 2022 at 20:03
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I am not a lawyer.

Fingerprints and other biometric markers are different on true identical twins.

There is at least a 2% chance and upto a 25% chance that one of the twins is a chimera. That means that DNA from some parts of the body do not match DNA from other parts. Perform the Paternity test using DNA from many different spots on each twin. Hair, Blood, Sputum, Skin and other places if at all possible such as the Kidneys or Liver.

The Impossible Crime (Unless You Know About Chimeras) A man accused of a crime while he was behind bars. Women told they are not the mothers of children they gave birth to. Has DNA gone mad? https://www.mcgill.ca/oss/article/critical-thinking/impossible-crime-unless-you-know-about-chimeras

Counter position but very very very old. MINNESOTA JOURNAL OF LAW, SCIENCE & TECHNOLOGY VOLUME 14 WINTER 2013 ISSUE 1 Essay: Chimeric Criminals by David H. Kaye https://conservancy.umn.edu/bitstream/handle/11299/144217/Chimeric-Criminals-by-David-Kay_MN-Journal-Law-Science-Tech-Issue-14-1.pdf

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    true, but the odds that they are a chimera in their testicles, the only part that would affect child DNA, is much lower. Still your right there are times that identical twins can be told apart, this question is more for the case when there aren't.
    – dsollen
    Commented Nov 29, 2022 at 18:24
  • If the twin left fingerprints or other markers, then that would work.
    – rjt
    Commented Nov 29, 2022 at 19:01
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    I'm not disputing that your answer states true facts, but it is non-responsive to the question. Also, while it is indeed possible to distinguish even identical twins with biomarkers, the cost of a test of sufficient quality to do so is on the order of tens of thousands of dollars (v. $100-$500 for an ordinary quality paternity DNA test) in a case over child support payments of perhaps hundreds of dollars per month. Worth the money in a murder prosecution, but not in a paternity case in most circumstances.
    – ohwilleke
    Commented Nov 29, 2022 at 21:33
  • So if this question was on Biology, the answer'd be a +$10k test?
    – Mazura
    Commented Nov 30, 2022 at 18:46

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