Opinion ID: 2389693
Heading Depth: 1
Heading Rank: 2

Heading: Enabling Jury to Use Its Own Estimate of Guilt Along with Expert's Calculations of Probability of Paternity

Text: For different reasons, however, we agree that the admission of the probability of paternity opinion as presented in this case was error. Although the jury learned, in cross-examination of the expert, of the expert's assumption of a 50% prior probability that defendant was the father, the clear impression given by the expert was that it was somehow a scientific assumption, an accepted part of a scientific calculation, objective, neutral, fair. It is no such thing  although it is often, indeed apparently almost regularly, used by forensic experts testifying in paternity matters. See Mikel Aichin, Some Fallacies in the Computation of Paternity Probabilities, 36 Am.J.Hum.Genetics 904, 906, 915 (1984) [Aichin, Fallacies ] (asserting that although most paternity testers accept mathematical framework underlying probability of paternity percentage, courts have gone too far in accepting this expert testimony). While counsel could have demonstrated this inherent lack of neutrality through fuller cross-examination, we think that his objections to the introduction of the probability of paternity percentage on that ground were well founded, fairly clearly stated, and should have been sustained. More than that, we conclude that even if not objected to sufficiently by counsel, the expert's opinion on probability of paternity did not satisfy the most fundamental requirement of expert testimony: its ability to aid the jury in its deliberations. See State v. Kelly, 97 N.J. 178, 209, 478 A. 2d 364 (1984). Moreover, as presented, the testimony create[d] [a] substantial danger of ... misleading the jury. Evid.R. 4; see also Fed. R.Evid. 403 (requiring exclusion of evidence where probative value substantially outweighed by the danger of ... misleading the jury). In this criminal case, the jury had no idea what to do with the probability of paternity percentage if its own estimate of probabilities (the prior probability of paternity as estimated by the jury apart from blood and tissue tests) was different from .5. There was neither guidance from the expert nor specific instructions from the trial court regarding this crucial aspect of the probability of paternity opinion. Was the jury supposed to reject the expert's opinion, or was that opinion still of scientific value because of its alleged objectivity? Was the expert's opinion valid even if the jury disagreed with the assumption of .5? If the jury concluded that the prior probability was .4 or .6, for example, the testimony gave them no idea of the consequences, no idea of what the impact (of such a change in the prior probability) would be on the formula that led to the ultimate opinion of probability of paternity. The jury did not know, for instance, that even if it believed that the prior probability was half of the assumed prior probability, namely, .25 instead of .5, the formula would not at all result in cutting in half the ultimate result, the probability of paternity percentage. Indeed, it would still have left the probability of paternity above 90% (the reduction apparently being from 96.55% to 90.24%). This total lack of neutrality on the part of the assumption could not be understood without that kind of information, which would have given the jury at least some basis for evaluating the significance of its own prior-probability determination, assuming it made one. Additionally, a full exposition of the impact of differing prior-probability assumptions would have aided the jury in evaluating the validity and usefulness of the formula itself as applied to paternity cases. There is no contention in this case by defendant that the probability of paternity thus computed is inadmissible as such (at oral argument defense counsel said that the probability of paternity opinion was admissible if the jury itself found that the prior probability was .5). We therefore could conclude this opinion with the implicit holding above, namely, that a probability of paternity opinion is admissible but only if the expert notes that the calculations leading to that opinion use as one of the critical factors an assumed prior probability of paternity of .5. While this .5 assumed prior probability clearly is neither neutral nor objective, rather than prohibiting an expert witness from describing it as such, we would leave it to counsel to challenge this characterization through cross-examination. However, a jury should be required to use its own estimate of the prior probability of paternity, one based on all of the evidence in the case other than the scientific evidence arising from the blood and tissue tests; a prior probability, in other words, based on facts of which the expert has absolutely no knowledge  the facts of the case as they would exist were there no scientific tests, no scientific reports, and no expert. Furthermore, the expert's testimony should be required to include an explanation to the jury of what the probability of paternity would be for a varying range of such prior probabilities, running, for example, from .1 to .9. On this last point, we note that a similar approach, initially suggested by Professors Ellman and Kaye, Probabilities and Proof, supra, 54 N.Y.U.L.Rev. at 1152-58, has been adopted by the Supreme Court of Oregon. Plemel v. Walter, 303 Or. 262, 735 P. 2d 1209, 1219 (1987). In that case, the court held that an expert who testifies regarding a probability of paternity should present calculations based on [varying] assumed prior probabilities of 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 percent. 735 P. 2d at 1219. Such an approach ensures that the jury's attention will be focused on the other evidence in the case and that it will not be misled by the expert's assumption of a prior probability of .5. Ibid.; see generally Kaye, Plemel As Primer, supra, 24 Willamette L.Rev. 867. Other courts have challenged the use of the fifty-fifty assumption. See, e.g., County of Sonoma v. Grant W., supra, 184 Cal. App. 3d at 868, 229 Cal. Rptr. at 301-302; Everett v. Everett, supra, 150 Cal. App. 3d at 1070, 201 Cal. Rptr. at 361; Commonwealth v. Beausoleil, 397 Mass. 206, 490 N.E. 2d 788, 797 n. 19 (1986); Kofford v. Flora, 744 P. 2d 1343, 1351-52 (Utah 1987); Bridgeman v. Commonwealth, 3 Va. App. 523, 351 S.E. 2d 598, 603 (1986); In re Paternity of M.J.B., supra, 425 N.W. 2d at 409. Indeed some courts have rejected the use of the probability of paternity statistic altogether on the grounds that the assigned .5 prior probability renders the statistic unreliable. E.g., Plemel v. Walter, supra, 303 Or. 262, 735 P. 2d at 1219; Cole v. Cole, 74 N.C. App. 247, 328 S.E. 2d 446, 450-51 (1985), aff'd 314 N.C. 660, 335 S.E. 2d 897 (1985); People v. Pasko, supra, 132 Ill.Dec. at 727, 540 N.E. 2d at 467; Sara H. v. Bart D., 121 Misc. 2d 425, 432, 467 N.Y.S. 2d 1001, 1005 (Fam.Ct. 1983). In this somewhat abstruse area, therefore, procedures should be designed to avoid an outcome based on an unchallenged and unexplained .5 assumption of prior probability  unexplained and unchallenged because of the possible lack of knowledge of counsel. And these procedures should also obviate a jury unenlightened concerning the impact, on the probability of paternity opinion, of varying prior probabilities. Other problems, such as the possibility of error in the lab tests, are common to all similar expert testimony and presumably will be brought out in cross-examination as they were here. [4]