Twin Research Volume 5 Number 5 pp. 488–492
intact pairs born in Minnesota between 1936 and 1955. In addi-
tion, the Registry includes 901 twin pairs born in Minnesota
from 1904 to 1934, as well as 391 male pairs born in
Minnesota from 1961 to 1964. The research focus is primarily
on human individual differences assessed by self-report.
Questionnaires completed by the participants include mea-
sures of personality, occupational interests, demographics, and
leisure-time activities. We outline major contributions that have
resulted from Registry research, as well as current and future
he Minnesota Twin Registry is a birth-record-based twin
registry. Begun in 1983, it includes data for 4307 surviving
The Minnesota Twin Registry, hereinafter referred to as the
Registry, is a birth-record-based twin registry. Begun in
1983, it includes data for 4307 surviving intact pairs born
in Minnesota between 1936 and 1955. In addition, the
Registry includes 901 twin pairs born in Minnesota from
1904 to 1934, as well as 391 male pairs born in Minnesota
from 1961 to 1964
Recruitment began with permission from the State Health
Department to photocopy all birth certificates reporting
multiple births. This photocopying process was made nec-
essary because Minnesota birth records prior to 1959 had
not been computerized. The information contained in
Minnesota birth records varies slightly from year to year.
Typically, however, it includes the child’s name and sex, the
name, age, race, and occupation of the father, the full
maiden name, age, race, and occupation of the mother, the
date, time, and place of birth, the number of living chil-
dren born previously to this mother, whether this was a
multiple birth and the birth order of this child, the birth
weight and length, and the name of the attending physi-
cian, midwife, or other informant. Sets of records including
one or more stillbirths or infant deaths and records of ille-
gitimate births were discarded.
The next step was to search the state’s death records for
the 6 months following the birth in order to eliminate pairs
that did not survive early infancy. Then, if the twins’
parents were likely to still be living, attempts were made to
locate them by using telephone directory listings beginning
in the area where the twins were born. If this did not work,
other people in the area with the same surname were con-
tacted; they were often relatives. Directories organized by
location and address were also used to identify persons now
living at the parents’ former address; these people some-
times knew where the family of the twins had gone, or
knew a long-term neighbor who might know. Other possi-
bilities included the father’s employer or the birth hospital.
When attempting to locate the adult twins directly,
marriage records were used to identify female name
changes. Then the state’s driver’s license records were gener-
ally searched. These techniques are less effective when the
twins have moved out of state. Still, twin pairs representing
about 80% of the intact pairs were located. Beyond this
percentage, the cost and effort involved in locating addi-
tional pairs became prohibitive.
Once located, the Registry mailed the potential partici-
pants a 4-page Biographical Questionnaire (BQ) along with
an introductory newsletter and a copy of a letter signed by
Minnesota’s governor urging participation. Sixty to 70% of
individuals responded within 8 weeks. Those that did not
respond were then contacted by telephone. Many of these
persons had mislaid the questionnaire and were happy to
complete a second copy when it was sent to them. Others
still had the first copy and the phone call provided the
incentive necessary for them to complete it. Two telephone
prompts, 6 to 8 weeks apart, generally yielded return of the
BQ from about 80% of the twins originally contacted.
On the second (and final) telephone prompt, twins
who had not returned the BQ were originally told that
their names would be entered in a lottery with a prize of
$1,000 if both members of their pair returned the ques-
tionnaire. This incentive was used only with the minority
of reluctant twins who had indicated that they would not
participate without some compensation. Thus the total
sample consists of those who would normally participate in
such studies, plus some additional participants who other-
wise would not have been sampled. It is therefore more
representative of the general population than many earlier
volunteer twin studies.
The lottery was later abandoned because it was found
to be illegal in many jurisdictions. It was replaced by direct
payment of $10 for participation to those who had not
responded by the second telephone prompt.
The Minnesota Twin Registry:
Current Status and Future Directions
Robert F.Krueger and Wendy Johnson
University of Minnesota,Twin Cities Campus,USA
Address for correspondence: Robert F. Krueger, Director, Minnesota
Twin Registry, Department of Psychology, University of Minnesota,
N414 Elliott Hall, 75 East River Road, Minneapolis, MN 55455-0344,
USA. Email: firstname.lastname@example.org.
The initial request for information from the partici-
pants was rather modest, on the assumption that they
would be more likely to continue to participate if requests
for information were never time-consuming or onerous at
any one time. This meant, however, that the Registry was
dependent on maintaining participant interest in providing
follow-up information for accumulation of a reasonable
database. To accomplish this, each participant was mailed a
computer-generated personalized report of his or her scores
on about 60 factor-analytically derived dimensions of per-
sonality and interest. Newsletters containing reports on
Registry activities and other information of interest to
twins are mailed at irregular intervals to all participants.
Parents, spouses, same-sex siblings, and offspring of the
Registry twins were also recruited, and their data are main-
tained in the Registry database, which also includes twins
whose co-twin did not elect to participate. The family-
member recruitments were based on the information
provided by the twins in the BQ. Targets were parents
when both were alive and well, same-sex singleton siblings
when there were at least 2, and same-sex offspring of the
twins (that is, same-sex pairs of cousins) aged at least 17.
About 69% of spouses contributed data. Initially, the par-
ticipation materials were distributed to the twins for relay
to their families, but later experience showed that it was
more effective to send the participation materials to the
family members directly.
Zygosity determination was accomplished using self-
reported answers to 5 questions in the BQ. These questions
referred to similarity of eye and hair color, overall similarity
in appearance, and the difficulty others encountered in dis-
tinguishing the twins. These questions were scored with
positive points for responses indicating similarity and nega-
tive points for responses indicating differences. Pairs with
mean scores of 4 or higher on these questions were classi-
fied as monozygotic (MZ) and all others were classified as
dizygotic (DZ). Zygosity was also determined serologically
for a subgroup of 74 pairs; the questionnaire approach was
96% accurate for this group.
Major Research Focus
The research focus in organizing the Registry is broad:
human individual differences as assessed by self-report and
collected by mail in order to maximize participation at
minimal cost. The initial BQ’s mailed to participants in the
mid-1980s included information about educational level,
occupation and income, marital status, offspring, composi-
tion of families, basic health, birth weight, gestational age,
maternal age at birth of twins, height, weight, and shoe size.
Since then, many subsequent questionnaires have been
completed by subsets of the twins. These include the
Multidimensional Personality Questionnaire (MPQ;
Tellegen, 1982), and questionnaires regarding occupational
interests, leisure-time activities, talents, environmental assets
(things such as reading materials and sports equipment
available in the twins’ homes currently and as children),
mental habits, attitudes and opinions, handedness, closeness
to twin, and moods. Registry subgroups have also com-
pleted specific questionnaires for special projects.
The birth cohort from 1904 to 1934 has completed a
self-report assessment of depression symptomatology. A
subgroup completed a follow-up in-person interview that
lasted about 5 hours and included extensive assessment of
intellectual functioning. Research with this birth cohort is
focused on changes in personality and intellectual function-
ing with age.
The male birth cohort from 1961 to 1964 completed a
2-hour structured telephone interview that included a 35-
item vocabulary test. Research with this birth cohort is
focused on family demographics, environment, and rela-
tionships, and illegal and antisocial activities of the
Our research approach is integrative: Registry data are
combined and/or compared with those from other samples
maintained at the University of Minnesota as available and
appropriate. For example, Registry data are often combined
and/or compared with those from the Minnesota Study of
Twins Reared Apart (MISTRA; Bouchard, 1994) and the
Minnesota Twin Family Study (MTFS; Iacono et al., 1999;
see also Iacono et al., this issue).
The initial work with Registry data was based on the BQ,
as it provided the first data available. One of the first ques-
tions examined concerned the effects of the various
incentives used to recruit Registry participants (Lykken et
al., 1990). No significant differences were found in educa-
tion, socioeconomic status (SES), or a variety of personality
and interest factors between twin pairs concordant for ease
of recruitment and those discordant. This suggested not
only that the recruitment techniques used had not created
distorting subgroups within the Registry sample, but also
that in general, selection bias may not be a problem in
research with adult twins when contacts are only by mail.
Other initial questions examined included changes in
neonatal mortality during the 45-year period from 1936 to
1981, height, weight, and body mass index (Carmichael &
McGue, 1995), and the heritability of SES, educational
attainment, fecundity, and risk of divorce.
Personality in General
The MPQ was the first questionnaire issued to Registry
participants after the BQ, and it formed the core of the
research program in the late 1980s and early 1990s. Much
of this program was motivated by hypotheses generated by
MISTRA, and these hypotheses required testing on large
representative populations of middle-aged twins. Using
both MISTRA and Registry data, Tellegen et al. (1988)
established that the four-parameter biometric model
including additive and nonadditive genetic components
and shared and nonshared environmental components was
necessary to provide an adequate fit for all of the 11
primary MPQ scales and their 3 higher-order factors.
Heritabilities of the scales ranged from .39 to .58, and the
shared environmental component was negligible for all but
2 of the scales. Three scales showed significant nonadditive
genetic effects. This paper both established that the
Registry and the MPQ would produce results consistent
Twin Research October 2002
The Minnesota Twin Registry: Current Status and Future Directions
with those of prior studies, and set the direction for the
next several studies based on the Registry.
The importance of the nonadditive genetic component
in biometric modeling of the MPQ scales and other traits
measured in MISTRA and the Registry suggested that
some traits are “emergenic” (Lykken, 1982; Lykken et al.,
1993). That is, such traits are determined by the interaction
of genetic influences rather than their sum. Thus, geneti-
cally identical MZ twins will tend to show high
concordances for these traits while DZ twins may be no
more similar than unrelated persons, and the combination
of genes may be rare enough that the traits do not “run in
families” even though they are highly genetic.
Extensions of Personality
This concept was extended from personality to interests and
leisure-time activities as data from those questionnaires
became available in the early 1990s. Lykken et al. (1993)
established that about 50% of variance in interests was associ-
ated with genetic variation, and Waller et al. (1995) suggested
that occupational interests, leisure-time activities, and person-
ality are psychometrically separate domains, though they are
related in meaningful ways. In the process, they introduced
new methods for ipsative scaling and nonmetric multidimen-
sional scaling, and proposed a model in which genetically
influenced precursor traits of aptitude and personality guide
the development of interests through the mechanisms of gene-
environment interaction and correlation.
Sex differences in heritability of MPQ personality scale
measures have also been investigated using the Registry.
Finkel and McGue (1997) found significant sex differences
in heritability for the Alienation, Control, and Absorption
scales, as well as significant nonadditive genetic influence
for all scales except Traditionalism and Absorption. A small
subgroup of Registry participants completed the MPQ
again, 10 years after the first completion. The average age
was 20 at first completion and 30 at follow-up. Their data
(McGue et al., 1993) showed significant mean decreases in
Negative Emotionality, increases in Constraint, but no sig-
nificant change in Positive Emotionality. Biometric analyses
suggested that a decrease in the variance in Negative
Emotionality was due to diminishing genetic influences,
stability in personality was due to genetic influences, and
personality change in adulthood largely reflected environ-
Specific Aspects of Personality
Research efforts have also focused on specific aspects of per-
sonality. Using the Well-Being scale of the MPQ, Lykken
and Tellegen (1996) measured happiness. The data showed
that SES, educational attainment, family income, marital
status, and religious commitment each failed to explain
more than 3% of the variance in Well-Being. From 44-
52% of the variance in Well-Being was associated with
genetic variation, however. Based on retests of smaller
samples after intervals of about 4.5 and 10 years, they con-
cluded that the heritability of the stable component of
Well-Being approaches 80%. This led to Happiness: What
Studies on Twins Show Us About Nature, Nurture, and the
Happiness Set-Point (Lykken, 2000b), in which Lykken
described how nature and nurture work to affect our sense
of well-being, and how people can work effectively to influ-
ence their own happiness.
Another aspect of personality that has been studied
using Registry data is Authoritarianism, which has histori-
cally been assumed to be the result of influences in the
rearing environment. As described by McCourt et al.
(1999), Registry and MISTRA data suggested that genetic
factors accounted for about 50% and non-shared environ-
mental factors accounted for about 35% of the variance in
Authoritarianism, with either shared environment or assor-
tative mating accounting for the remainder. Purportedly
relevant environmental variables were associated with the
Authoritarianism scores of the individuals reared by biolog-
ical relatives, but this was not true for adoptees.
Effects of Aging
The 1904–1934 Registry cohort has been used primarily to
investigate effects of aging on cognitive function. Among
other projects, Finkel and McGue (1994) estimated that
55% of the variance in memory performance was genetic.
They found that the relationship between memory and
cognitive variables was primarily genetic in nature, while
the relationship between memory and lifestyle variables was
environmental in nature. Finkel and McGue (1998) found
no age differences in the heritability of memory nor in the
genetic and environmental mediation of the correlations
between memory and other cognitive factors, though they
did find age differences in the correlations themselves. This
suggested that the natures of the relationships between
memory and other cognitive factors were stable across age,
though the extent of their relationships were not. Finkel et
al. (1995) used 12 physiological, cognitive ability, and pro-
cessing speed variables to describe functional age as a
regression equation that predicted 66% of the variance in
chronological age. The twin similarity data suggested that
the relative genetic and environmental influences on the
three principal components of functional age varied greatly.
MISTRA data suggested the possibility that the occurrence
of major life events has a heritable component. This was
investigated with respect to divorce (McGue & Lykken,
1992) using Registry data. Concordance for divorce was
significantly higher in MZ twins than in DZ twins, sug-
gesting genetic influence. In addition, the family
background of both spouses contributed independently to
couples’ risk of divorce, suggesting that divorce may com-
monly result from characteristics that each spouse brings to
the marriage, rather than interaction effects. The specific
aspects of personality contributing to divorce were investi-
gated by Jockin et al. (1996). Positive and Negative
Emotionality were positively related to risk of divorce,
while Constraint was negatively related.
On the other hand, Registry data suggested that mate
selection is not the result of heritable characteristics, either
similar or dissimilar between spouses (Lykken & Tellegen,
1993). Neither spousal similarity nor idiosyncratic criteria
among 74 mainly psychological variables could account for
specific mate selection. A subgroup of the full sample also
rated their attraction to their twin’s mate or to their spouse’s
twin. The findings suggested that characteristics of both the
Twin Research October 2002
Robert F. Krueger and Wendy Johnson
chooser and the chosen constrain mate selection only very
weakly. Lykken and Tellegen proposed that romantic infat-
uation determines the final choice of mate from a broad
field of potential eligibles, and that this phenomenon is
Personality Structure and Links to Social Behavior
Recent areas of investigation using Registry data concern
the structure of personality and links to social behavior.
Using the Registry, Krueger (2000) showed that the under-
lying genetic and environmental structure of the MPQ
scales mirror their phenotypic structure. He discovered
Positive Emotionality, Negative Emotionality, and
Constraint dimensions in patterns of phenotypic, genetic,
and environmental correlations among MPQ scales.
Extending this work, the etiologies of altruism and antiso-
cial behavior were investigated by Krueger et al. (2001).
Findings suggested that altruism and antisocial behavior are
uncorrelated tendencies arising from different sources.
Altruism was linked primarily to shared and nonshared
environmental influences and MPQ Positive Emotionality,
while antisocial behavior was linked primarily to genes,
nonshared environments, MPQ Negative Emotionality and
lack of MPQ Constraint.
Registry data have also been used to study the
antecedents and correlates of antisocial behavior. Lykken
(2000a) reported twin correlations along with mean scores
and associated effect sizes for those scoring in the highest
25% and the lower 75% on a series of variables measuring
MPQ Negative Emotionality and Constraint along with
aspects of lack of socialization. The lack of socialization
variables included admissions of illegal and antisocial activi-
ties, antagonistic relationships with parents, delinquency
and violence of friends, lack of religious commitment, and
lack of parental commitment during youth.
A wealth of additional Registry data have already been
accumulated and await further analysis. Currently, there are
projects in progress involving changes in MPQ scale scores
across the lifespan, the inter-relationships of marriage and
personality, and the personality antecedents of leadership
behavior. Other projects in the planning phases include the
development of masculinity and femininity scales based on
interest inventories and longitudinal analysis of MPQ scale
scores in participants over the age of 55.
In addition, we plan to use the Registry in a psychobio-
logical investigation of the externalizing dimension of
psychopathology discussed by Krueger (1999; Krueger et al.,
2002). The project is based on the idea that externalizing is a
broad vulnerability that underlies psychopathological syn-
dromes involving antisocial behavior and substance
dependence as well as personality traits reflecting impulsivity
and disinhibition. The first specific objective is to develop
and operationalize a hierarchical model of externalizing
behavior that defines the core vulnerability construct and its
distinctive behavioral manifestations from the normal
through the abnormal range. This involves development of a
self-report instrument to measure externalizing characteris-
tics of individuals on a broad continuum. After initial
development and refinement, the inventory will be mailed to
Registry participants. The data they provide will make it possi-
ble to confirm the psychometric properties of the externalizing
inventory in a general population sample, as well as to assess
the extent to which the self-report externalizing inventory
maps underlying genotypic externalizing structures.
It is almost 20 years since work began to establish the
Minnesota Twin Registry. In that time, the Registry has
yielded a large volume of psychological data and important
research findings. The Registry remains a valuable research
tool with bright prospects for important future contribu-
tions to our understanding of human individual differences.
Robert F. Krueger was supported by National Institute of
Mental Health Grant MH65137. The establishment of the
Minnesota Twin Registry was supported by National
Institute of Mental Health Grant MH37860 to David
Lykken. We thank David Lykken for his helpful comments
on a previous draft of this article.
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Robert F. Krueger and Wendy Johnson
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