DEMONSTRATING THE VALIDITY OF TWIN RESEARCH IN CRIMINOLOGY

• Author: JOHN PAUL WRIGHT,JOSEPH L. NEDELEC,ERIC J. CONNOLLY,J. C. BARNES,JOSEPH A. SCHWARTZ,KEVIN M. BEAVER,BRIAN B. BOUTWELL
• DOI: http://doi.org/10.1111/1745-9125.12049
• Published date: 01 November 2014
• Date: 01 November 2014

DEMONSTRATING THE VALIDITY OF TWIN

RESEARCH IN CRIMINOLOGY∗

J. C. BARNES,1JOHN PAUL WRIGHT,1,6 BRIAN B. BOUTWELL,2

JOSEPH A. SCHWARTZ,3ERIC J. CONNOLLY,4

JOSEPH L. NEDELEC,1and KEVIN M. BEAVER5,6

1School of Criminal Justice, University of Cincinnati

2School of Social Work, Saint Louis University

3School of Criminology and Criminal Justice, University of Nebraska at Omaha

4Criminal Justice Department, Pennsylvania State University, Abington

5College of Criminology and Criminal Justice, Florida State University

6Center for Social and Humanities Research, King Abdulaziz University,

Jeddah

KEYWORDS: assumptions, behavior genetics, biosocial, empirical, quantitative, twins

In a recent article published in Criminology, Burt and Simons (2014) claimed that

the statistical violations of the classical twin design render heritability studies useless.

Claiming quantitative genetics is “fatally flawed” and describing the results generated

from these models as “preposterous,” Burt and Simons took the unprecedented step

to call for abandoning heritability studies and their constituent findings. We show that

their call for an “end to heritability studies” was premature, misleading, and entirely

without merit. Specifically, we trace the history of behavioral genetics and show that 1)

the Burt and Simons critique dates back 40 years and has been subject to a broad array

of empirical investigations, 2) the violation of assumptions in twin models does not in-

validate their results, and 3) Burt and Simons created a distorted and highly misleading

portrait of behavioral genetics and those who use quantitative genetic approaches.

“The flaws of twin studies are not fatal, but rather seem no worse (and may be better)

than the flaws of the typical causal study that relies on observational data.”

(Felson, 2012: ii)

Behavioral genetic research has existed for more than 100 years (Maxson, 2007). Since

its inception, it has been a lightning rod of criticism, especially by scholars who are inal-

terably opposed to linking biology with behavior. Over this time, numerous critics (e.g.,

∗Additional supporting information can be found in the listing for this article in the Wiley Online

Library at http://onlinelibrary.wiley.com/doi/10.1111/crim.2014.52.issue-4/issuetoc.

We wish to thank each of the five anonymous reviewers for their scholarly insight. In addition, we

would like to acknowledge Mara Brendgen, Francis Cullen, Lisabeth DiLalla, Christopher Fer-

guson, Judith Rich Harris, Kenneth Kendler, Terrie Moffitt, Christopher Patrick, Steven Pinker,

Stephen Tibbetts, Catherine Tuvblad, and Anthony Walsh for their comments, suggestions, and

feedback on previous drafts of this article. Of course, all errors and omissions are ours and ours

alone. Direct correspondence to J. C. Barnes, School of Criminal Justice, University of Cincinnati,

Cincinnati, OH 45221 (e-mail: jc.barnes@uc.edu).

C2014 American Society of Criminology doi: 10.1111/1745-9125.12049

CRIMINOLOGY Volume 52 Number 4 588–626 2014 588

VALIDITY OF TWIN RESEARCH 589

Joseph, 2004; Lewontin, Rose, and Kamin, 1984) have leveled various charges against

twin designs and the assumptions on which they are based. Beginning in the 1970s, polit-

ically motivated critics of behavioral genetics launched an all-out crusade against such

methods, the findings emanating from them, and even on the researchers themselves

(Segerstr˚

ale, 2000). These critics called for an end to the idea that biology had anything

to do with behavior, noting that sociobiology was a “dangerous idea.” Relying heavily

on anecdotes, opinions, and the occasional mathematical example, critics of behavioral

genetics were unrelenting in their attack.

In response, a small but growing force of behavioral geneticists, statisticians, and other

scholars launched a prolonged effort to collect larger samples of twins, other geneti-

cally related relatives, and adoptees. They used these samples to test, retest, and re-

fine behavioral genetic models. Throughout the 1980s, behavioral geneticists published

study after study documenting the robustness of behavioral genetic methods, especially

those designed to assess heritability (Floderus-Myrhed, Pederson, and Rasmuson, 1980;

Paul, 1980; Pederson et al., 1985; Rice, Cloninger, and Reich, 1980; Rushton et al., 1986;

Scarr, Scarf, and Weinberg, 1980). These studies quickly multiplied, eventually leading

behavioral geneticists to “lose their identity” because they became so integrated within

psychology and other fields (Scarr, 1987). By the end of the 1980s, the war was over with

a large body of research findings supporting the general thrust of behavioral genetic mod-

eling (Plomin and Bergeman, 1991). As time went on, isolated critics (e.g., Joseph, 2004)

emerged only to be greeted by even more empirical evidence in favor of the validity of

the findings stemming from behavioral genetic studies. Today, behavioral genetic stud-

ies inform a broad range of fields, including medicine, psychiatry, psychology, education,

and even criminology. Summarizing the large body of behavioral genetic findings that had

emerged prior to 2002, Pinker (2002: 374) observed:

The results [of heritability studies] come out roughly the same no matter what is

measured or how it is measured. . .. All of this translates into substantial heritability

values, generally between .25 and .75. A conventional summary is that about half of

the variation in intelligence, personality, and life outcomes is heritable.

The collective body of biosocial evidence thus directly aligns with the “reality” of find-

ings from every other discipline. Broadly speaking, biosocial criminologists have found

that approximately 50 percent of the variance in antisocial phenotypes can be attributed

to genetic influences, followed by unique environmental influences and, to a lesser extent,

common (or shared) environmental influences (Beaver, 2013; Ferguson, 2010; Moffitt,

2005).

These findings, and others, however, find themselves once again imperiled. A recent

publication in Criminology by Burt and Simons (2014; hereafter Burt and Simons) called

for criminologists to abandon heritability studies and effectively to jettison them from

the discipline. In doing so, Burt and Simons called for a de facto form of censorship.

Their logic was straightforward: Twin studies are fatally flawed, the findings cannot be

trusted, old findings should be placed on the scrapheap of scientific history, and new find-

ings only reify what is known to be untrustworthy. As a result, no bona fide criminolog-

ical journal should publish twin-based research moving forward. Joining history’s critics

(Joseph, 2004), Burt and Simons critiqued heritability studies and those who use behav-

ioral genetics models. Although their arguments were multifaceted, they bear a striking

590 BARNES ET AL.

resemblance to the arguments leveled against behavioral geneticists in the 1970s. Even so,

their argument was powerful, emotionally appealing, and seductive, especially for those

uninformed about behavioral genetics and for those ideologically opposed to biology.

Burt and Simons argued that behavioral genetic methods are fundamentally flawed

because violation of “crucial assumptions” and “technical limitations” inevitably leads

to upwardly biased estimates of heritability and to downwardly biased estimates of the

common environment. Burt and Simons detailed a series of methodological arguments

against heritability studies, arguing, for instance, that violations of the equal environ-

ments assumption (EEA; “the environment of MZ co-twins is no more similar than that

of DZ co-twins”) are “flatly contradicted by both empirical evidence and common sense”

(p. 231).1Indeed, they even told us “behavioral geneticists acknowledged that the EEA

was invalid” (p. 232). Yet the EEA was just one of many behavioral genetic assumptions

attacked by Burt and Simons. Because twin research rests on critical assumptions, as-

sumptions Burt and Simons argued are always violated, findings from heritability studies

are “biologically nonsensical” (p. 225). After purportedly scrutinizing prior studies, Burt

and Simons went on to brand entire bodies of carefully collected and meticulously ana-

lyzed scientific evidence as useless, and even “preposterous” (p. 236). Burt and Simons

then proceeded to tell readers that behavioral genetic findings are “implausible” and that

behavioral genetic research rests on a “dubious foundation” (p. 223). In the end, they

claimed to have exposed the critical flaws of behavioral genetic research.

But did they expose any fundamental flaws of behavioral genetics? Did they present the

readers of Criminology a fair and impartial assessment of behavioral genetics research?

Did they delve deeply into the vast body of behavioral genetic literature, into the technical

aspects of assumption violation, biased parameter estimates, and erroneous conclusions?

Or, instead, did they join past critics and reify arguments already shown to be unsubstan-

tiated by empirical evidence? Clearly, as even Burt and Simons recognized, “most of the

arguments” in their article “are not original” (p. 225). Does repeating arguments origi-

nally proposed in the 1970s, and refuted shortly thereafter, make them relevant today? In

short, can we believe Burt and Simons have ascertained unbiased and definitive proof that

behavioral genetic models are wrong? Have they done something no other statistician,

theorist, or behavioral geneticist in the last four decades has been able to accomplish?

In the following pages, we show that the criticisms of behavioral genetic models ad-

vanced by Burt and Simons have not only been answered by dozens of prior studies but

also that they are wrong. We show this mathematically, with an in-depth examination

of the basis of twin designs, with our own simulation models that directly illustrate the

impact of violating assumptions on heritability estimates, and by examining 61 empirical

studies that have tested one of the “critical assumptions” pointed out by Burt and Simons:

the EEA. In the end, we show that the violations of behavioral genetic assumptions hardly

qualify as “fatal flaws” that can be used as a justification to abandon heritability studies.

Indeed, the evidence to this fact is overwhelming and should reduce any confidence read-

ers extended to the Burt and Simons article.

If showing mathematically that Burt and Simons grossly exaggerated their claims is

not sufficient, we then examine the scholarship underpinning their critique of heritability

studies. We show, in detail, where they misquoted scholars, where they misrepresented

1. MZ =monozygotic and DZ =dizygotic.