Opinion by ADKINS, J.
We have often said that when it comes to expert testimony, trial court judges protect juries from "junk science" and other unsupported scientific conclusions. But in the complex field of behavioral disorders, the task of determining whether causation testimony rests on sound science is fraught with potential pitfalls. This case presents the question of whether studies concluding that lead exposure causes various attention problems constitute a sufficient factual basis for an expert's testimony that lead caused the plaintiff's Attention Deficit Hyperactivity Disorder ("ADHD").
FACTS AND LEGAL PROCEEDINGS
Respondent Starlena Stevenson was born on December 22, 1990. For the first ten months of her life, Stevenson lived at 2110 Clifton Avenue in Baltimore City with her mother, Charlena Montgomery, and maternal grandmother, Lorena Cooks. In the fall of 1991, Stevenson and Montgomery moved to 3823 Fairview Avenue ("Fairview"), where they lived for 15 months. At the time, Fairview was owned in part by Petitioner Stanley Rochkind. According to Montgomery, Fairview contained chipping and flaking paint on the windowsills, floors, and front porch. Montgomery witnessed Stevenson mouthing the windows at Fairview. In early 1993, Stevenson and her mother moved to an apartment on Pennsylvania Avenue ("Pennsylvania").
In 1992 and 1993, Stevenson's blood lead level was tested three times. The results were as follows:
When Stevenson was five years old, she was evaluated by Thomas Ley, Ph.D., a psychologist with the Kennedy Krieger Institute, because she was struggling to pay attention in school. Dr. Ley found that Stevenson's cognitive functioning was within the "low average to borderline range." He diagnosed Stevenson with ADHD and recommended that she begin taking medication to treat it. Over the next few years, Stevenson was prescribed Dexedrine, Ritalin, and Adderall to treat her ADHD. In 2004, when she was thirteen years old, Stevenson attempted suicide by cutting her wrists and overdosing on prescription medication. The following year, Stevenson complained of auditory hallucinations and depression. She was evaluated by a psychologist at Mount Washington Pediatric Hospital ("MWPH") who diagnosed her with major depressive disorder and generalized anxiety disorder.
Since graduating from high school in 2008, Stevenson has been sporadically employed. She has worked as a patient transporter for the University of Maryland Medical System ("UMMS"), a cashier for Royal Farms, and a babysitter. Stevenson was fired from her job at UMMS, and she testified that she quit her job at Royal Farms because she was bored. As of October 2014, Stevenson was unemployed.
In December 2011, Stevenson filed suit against Rochkind in the Circuit Court for Baltimore City for negligence and violations of the Maryland Consumer Protection Act.
Before trial, Rochkind filed four motions in limine seeking to exclude Dr. Hall-Carrington's testimony. He argued that she should not be permitted to testify that Fairview was a source of Stevenson's lead exposure or that such exposure caused Stevenson's "cognitive deficits," including, specifically, ADHD. Rochkind requested a Frye-Reed hearing on each motion. The court denied his request. After hearing arguments on the motions in limine, the court denied them as well. The jury returned a verdict in favor of Stevenson, awarding her $829,000 in economic damages and $534,000 in noneconomic damages. Rochkind filed a motion for a new trial, or, in the alternative, a remittitur. The court granted his motion in part and ordered a new trial on the issue of damages alone.
The partial new trial began in October 2014. Before trial, Rochkind renewed his motions in limine to exclude Dr. Hall-Carrington's ADHD testimony, which were again denied. The court declined to hold a Frye-Reed hearing, explaining that Dr. Hall-Carrington's opinions are "not new science" or "new conclusions." It admitted her testimony under Maryland Rule 5-702 because it found that she drew from "reliable sources."
During trial, Dr. Hall-Carrington testified as to both general and specific ADHD causation. She explained that studies show that lead exposure can cause "attention problems[ ] or ADHD" generally. She also opined "within a reasonable degree of medical probability" that lead exposure caused Stevenson's ADHD specifically. To support her testimony, Dr. Hall-Carrington relied on a publication from the Environmental Protection Agency reviewing the most recent studies on the effects of lead exposure in children, titled "Integrated Science Assessment for Lead" ("the EPA-ISA"). She testified that the EPA-ISA concluded that there is a causal relationship between lead exposure and the symptoms of ADHD, such as attention decrements, impulsivity, and hyperactivity. Dr. Hall-Carrington also testified that "some years ago there was a concern with suicide in kids [taking] Adderall." In closing argument, Stevenson's counsel implied that Stevenson's depression and hallucinations were side effects of her ADHD medications, including Adderall.
The jury awarded Stevenson $753,000 in economic damages and $700,000 in noneconomic damages. Due to the statutory cap on noneconomic damages, the court reduced the total judgment to $1,103,000. See Md. Code (1986, 2013 Repl. Vol.), § 11-108 of the Courts and Judicial Proceedings Article. Rochkind filed a motion for a new trial, which the court denied. He filed a timely appeal.
The Court of Special Appeals held that the trial court did not err in failing to hold a Frye-Reed hearing on Dr. Hall-Carrington's general causation testimony because the studies she relied upon did not reach novel conclusions and "used methodologies that are generally accepted" in the scientific community. Rochkind v. Stevenson, 229 Md.App. 422, 464 (2016). The intermediate appellate court also held that the trial court properly admitted Dr. Hall-Carrington's specific causation testimony under Rule 5-702 because her opinion "was supported by an adequate factual basis and was sufficient to allow the jury to decide the causal connection, if any, between lead exposure and [ ] Stevenson's ADHD." Id. at 465. Rochkind appealed.
We granted certiorari to answer the following questions:
Because we answer the first question in the affirmative, we reverse the decision of the Court of Special Appeals and remand for a new trial on the issue of damages. Therefore, we do not reach the second question.
STANDARD OF REVIEW
Rochkind challenges the trial court's decision to admit Dr. Hall-Carrington's expert testimony on the causal relationship between lead exposure and ADHD. "[T]he admissibility of expert testimony is a matter largely within the discretion of the trial court." Bryant v. State, 393 Md. 196, 203 (2006) (citations and internal quotation marks omitted). Thus, we review a trial court's decision to admit or exclude expert testimony only for an abuse of discretion. Rollins v. State, 392 Md. 455, 499-500 (2006). "Such a ruling, however, may be reversed on appeal if it is founded on an error of law or some serious mistake, or if the trial court clearly abused its discretion." Sippio v. State, 350 Md. 633, 648 (1998) (citation and internal quotation marks omitted). Additionally, we will not affirm a decision within the discretion of the trial court if the judge acts in an "arbitrary or capricious manner" or "beyond the letter or reason of the law." Garg v. Garg, 393 Md. 225, 238 (2006) (citation omitted).
Rochkind argues that Dr. Hall-Carrington's testimony should have been excluded because it failed to meet the requirements of both Maryland Rule 5-702 and our Frye-Reed standard for testimony based on a novel scientific theory. Rule 5-702 governs the admissibility of all expert testimony. Reed v. State, 283 Md. 374 (1978), which adopted the standard set forth by the U.S. Court of Appeals for the D.C. Circuit in Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), added an additional requirement for expert testimony based on a novel scientific method. Under Frye-Reed, a trial court must determine whether a novel scientific method is generally accepted within the relevant scientific community before testimony based on that method can be admitted. Reed, 283 Md. at 381. Stevenson contends that Dr. Hall-Carrington's testimony was properly admitted because it clears both hurdles. We begin with Rule 5-702.
Maryland Rule 5-702
To determine whether expert testimony "will assist the trier of fact to understand the evidence or to determine a fact in issue," Rule 5-702 directs a trial court to evaluate "(1) whether the witness is qualified as an expert by knowledge, skill, experience, training, or education[;] (2) the appropriateness of the expert testimony on the particular subject[;] and (3) whether a sufficient factual basis exists to support the expert testimony." The burden rests with the proponent of the expert testimony to demonstrate that these requirements have been met. Bomas v. State, 412 Md. 392, 417-18 (2010).
We have interpreted the third prong of this analysis—sufficient factual basis—to include two subfactors: an adequate supply of data and a reliable methodology. Roy v. Dackman, 445 Md. 23, 42-43 (2015) (citation omitted); see also Exxon Mobil Corp. v. Ford, 433 Md. 426, 478 (2013). To constitute "more than mere speculation or conjecture," the expert's opinion must be based on facts sufficient to "indicate the use of reliable principles and methodology in support of the expert's conclusions." Ford, 433 Md. at 478 (citation and internal quotation marks omitted). To demonstrate a sufficient factual basis, an expert must establish that her testimony is supported by both subfactors.
The data supporting an expert's testimony "may arise from a number of sources, such as facts obtained from the expert's first-hand knowledge, facts obtained from the testimony of others, and facts related to an expert through the use of hypothetical questions." Sippio, 350 Md. at 653 (citation omitted). The facts or data that form the basis of the expert's opinion may be "perceived by or made known to the expert at or before the hearing." Md. Rule 5-703(a). If the materials the expert relies upon are "of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject," they do not need to be admissible in evidence. Id.
In addition to drawing from an adequate supply of data, an expert must use a reliable methodology to reach her conclusions. Exxon Mobil Corp. v. Albright, 433 Md. 303, 418-19 (2013) (citation omitted). To satisfy this prong, "an expert opinion must provide a sound reasoning process for inducing its conclusion from the factual data and must have an adequate theory or rational explanation of how the factual data led to the expert's conclusion." Ford, 433 Md. at 481 (citation and internal quotation marks omitted). We have explained that "for an opinion to assist a trier of fact, the trier of fact must be able to evaluate the reasoning underlying that opinion." Ross v. Hous. Auth. of Balt. City, 430 Md. 648, 663 (2013). Thus, conclusory statements of opinion are not sufficient—the expert must be able to articulate a reliable methodology for how she reached her conclusion. Ford, 433 Md. at 481-82 (citation omitted).
Rochkind argues that Dr. Hall-Carrington's specific causation testimony does not satisfy Rule 5-702(3) because she did not rule out other potential causes of Stevenson's ADHD or otherwise demonstrate a reliable methodology for her opinion.
Adequate Supply of Data
To support her testimony that lead exposure can cause ADHD, Dr. Hall-Carrington relied on the EPA-ISA paper, which explains that "multiple, high quality epidemiologic studies" have revealed "a causal relationship between [lead] exposure and attention decrements, impulsivity, and hyperactivity in children." U.S. Envtl. Prot. Agency, EPA/600/R-10/075F, Integrated Science Assessment for Lead 4-314 tbl. 4-17, 4-289 (2013) [hereinafter EPA-ISA], https://www.epa.gov/isa/integrated-science-assessment-isa-lead (follow "PDF" hyperlink) [https://perma.cc/ECQ7-TFWS]. During the second trial, Dr. Hall-Carrington testified that these studies found that children with blood lead levels similar to Stevenson's "can have attention problems, or ADHD, as well as learning disabilities." When asked if this portion of the EPA-ISA mentions ADHD specifically, she testified, "No, it doesn't." She continued, "It basically is listing the symptoms, so to speak, of ADHD." As to Stevenson's ADHD specifically, Dr. Hall-Carrington testified that it was caused by her exposure to lead.
We have yet to decide the extent to which epidemiological studies can support expert testimony on causation. Thus, we look to other jurisdictions for guidance. In General Electric Co. v. Joiner, 522 U.S. 136 (1997), the U.S. Supreme Court evaluated expert testimony relying on epidemiological studies for the conclusion that polychlorinated biphenyls ("PCB's") can cause lung cancer.
Dr. Hall-Carrington did not provide a sufficient factual foundation for why she thought the EPA-ISA supported her conclusion that lead exposure can cause ADHD. The studies described in the EPA-ISA finding a causal relationship between lead exposure and attention deficits and hyperactivity do not go that far. The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders ("the DSM-V") lists several criteria for an ADHD diagnosis, including that the patient exhibit a minimum number of specific symptoms "for at least [six] months to a degree that is inconsistent with [the patient's] developmental level and that negatively impacts directly on [the patient's] social and academic/occupational activities."
Although research shows that lead exposure can cause general attention deficits and hyperactivity, these lead-caused behaviors do not necessarily indicate that an individual has ADHD because these behaviors are also symptoms of a variety of other disorders and learning disabilities. See id. at 63-65; Nat'l Inst. of Mental Health, Attention-Deficit/Hyperactivity Disorder (ADHD): The Basics, https://www.nimh.nih.gov/ health/publications/attention-deficit-hyperactivity-disorder-adhd-the-basics/index.shtml (follow "Download PDF" hyperlink) [https://perma.cc/89RV-AGRQ]; Guckenberger v. Boston Univ., 974 F.Supp. 106, 131 (D. Mass. 1997). Indeed, the DSM-V instructs clinicians to rule out several other behavioral disorders with similar symptoms before diagnosing a child with ADHD. DSM-V at 63-65. Clearly, general attention deficits and ADHD are not one and the same.
The jump from attention deficits and hyperactivity to a clinical ADHD diagnosis may seem reasonable, but we have explained that "just because a conclusion is reasonable does not mean that a court must permit an expert to make it." Ross, 430 Md. at 664. Because of the added weight a jury might give to testimony from a designated expert, the trial court "ought to insist that a proffered expert bring to the jury more than the lawyers can offer in argument." Id. (citation omitted). In equating attention deficits and hyperactivity with a clinical ADHD diagnosis, Dr. Hall-Carrington painted an inaccurate picture of the scientific research regarding lead poisoning—she overstated the known effects of lead exposure. Her testimony suffers from the same "analytical gap" described in Joiner.
To be sure, the EPA-ISA includes discussion of studies exploring the relationship between lead exposure and ADHD specifically,
Indeed, the EPA-ISA explains that in declaring a causal relationship between lead exposure and attention deficits, impulsivity, and hyperactivity, studies finding a lead-associated increase in ADHD were "not a major consideration" because of "inconsistent consideration for potential confounding by factors such as [socioeconomic status], parental education, or parental caregiving quality." EPA-ISA at 4-289-90. The EPA-ISA also notes that ADHD is "considered to have a strong familial component" and that studies revealing an association between lead exposure and ADHD have been critiqued for failing to take parental history of ADHD into account. Id. at 4-151 (citations omitted). Thus, without accounting for potential confounding factors, including family history of ADHD, these epidemiological studies cannot support Dr. Hall-Carrington's general causation testimony.
Apart from the EPA-ISA, Dr. Hall-Carrington did not cite to any other studies to support her opinion that lead exposure can cause ADHD.
Without epidemiological studies—or other reliable evidence—demonstrating a causal link between lead exposure and ADHD, Dr. Hall-Carrington's testimony "amounted to no more than mere speculation and conjecture." Bentley v. Carroll, 355 Md. 312, 338 (1999). Thus, Stevenson failed to carry her burden of demonstrating that Dr. Hall-Carrington had an adequate supply of data to support her general causation testimony. Accordingly, her testimony is not admissible under Rule 5-702(3). We do not reach the question of whether Dr. Hall-Carrington utilized a reliable methodology to formulate her testimony.
In sum, the trial court failed to determine whether Stevenson's proffered sources logically supported Dr. Hall-Carrington's opinion that lead exposure can cause ADHD. In other words, it failed to check for an "analytical gap" between the expert's data and her conclusion. Because we find that the trial court clearly erred in this respect, we hold that it abused its discretion in admitting the expert testimony.
Rochkind argues that the trial court should have held a Frye-Reed hearing as to Dr. Hall-Carrington's general causation testimony because the existence of a causal link between lead exposure and ADHD is novel, and it is not generally accepted in the scientific community. Because we conclude that all of Dr. Hall-Carrington's ADHD causation testimony should have been excluded under Rule 5-702, we decline to reach this issue.
Dr. Hall-Carrington did not have a sufficient factual basis for her testimony as required by Rule 5-702(3). Thus, the trial court abused its discretion in permitting her to opine that lead exposure can cause ADHD generally and that lead caused Stevenson's ADHD specifically. Accordingly, we remand for the circuit court to hold a new trial on the issue of damages.
The FRE were restyled in 2011 without substantive change. FRE 702 now provides: