Also known as: prior probabilities · Bayesian prior
Written by attorneys · grounded in primary & secondary sources — see below
An initial probability assigned to a hypothesis before incorporation of new evidence. In parentage proceedings the Uniform Parentage Act fixes this value at 0.50 when genetic testing laboratories compute the probability of parentage from the combined relationship index.
Piper Patel alleges that Patrick Phan fathered her child. An accredited laboratory performs court-ordered genetic testing on the three individuals and reports a 99 percent probability of parentage together with a combined relationship index of 102 to 1. The court treats the results as identifying Patrick as the genetic parent because the laboratory applied the required prior probability of 0.50 in reaching those figures.
Paige Porter testifies that Preston Price is the father of her child. An expert applies Bayes' Theorem to HLA test results and begins the calculation with a prior probability of 0.50 that Preston is the father. The expert then updates that starting figure with the test data to produce a final probability of paternity that the jury may consider.
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The statute adopts the conventional starting assumption that the tested individual and a random unrelated man are equally likely to be the father before the genetic data are considered. This neutral baseline prevents the laboratory from injecting an unstated factual judgment into the calculation and ensures that the reported probability reflects only the test results.
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The court cannot verify that the reported probability meets the statutory thresholds. Without confirmation that a prior probability of 0.50 was applied, the numerical results fail to satisfy Unif. Parentage Act § 506(a) and do not identify the tested individual as a genetic parent.
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No. An identified individual may challenge the result only by presenting other genetic testing that either excludes the individual or identifies another possible genetic parent. Substituting a different prior probability does not constitute a valid statutory challenge.
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The laboratory multiplies the combined relationship index by the prior odds derived from the 0.50 prior probability. The resulting posterior probability must reach at least 99 percent and the index must reach at least 100 to 1 before the statute treats the tested individual as a genetic parent.
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