tacit coordination in monozygotic twins, dizygotic twins and virtual twins: effects and implications...

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Tacit coordination in monozygotic twins, dizygotic twins and virtual twins: Effects and implications of genetic relatedness Nancy L. Segal a, * , Shirley A. McGuire b , Steven A. Miller a , June Havlena a a California State University, Psychology Department, 800 N. State College Building, Fullerton, CA 92834, United States b University of San Francisco, Psychology Department, San Francisco, CA, United States article info Article history: Received 30 May 2008 Received in revised form 17 June 2008 Accepted 23 June 2008 Available online 3 August 2008 Keywords: Twins Virtual twins Coordination Cooperation abstract Game theorists have long been interested in why some partners are more successful at achieving mutual goals than others. The present study was conducted to determine if tacit coordination (non-negotiated consensus) varies as a function of genetic relatedness between social partners. The sample included 7–13-year-old monozygotic (MZ) twin pairs (n = 53), dizygotic (DZ) twin pairs (n = 85) and virtual twin (VT) pairs (n = 42; same-age unrelated siblings). Co-twin agreement on questions completed under indi- vidual and coordination conditions was compared across the three groups. MZ twins showed significantly greater overall agreement than DZ twins and VTs, using multiple criteria. The implications of these find- ings are considered with reference to behavioral-genetic and evolutionary theories of behavior. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Game theorists have a continuing interest in why some partners are more likely to achieve mutual goals than others (Sugden & Zammarrón, 2006). There has been increased appreciation for the complexity of social environments in which most decisions are made. Variables of interest include individuals’ emotional state and sensitivity to the intentions of others. The recent efforts of neuroscientists and economists have defined the neuroeconomic approach, aimed at identifying brain mechanisms associated with various classes of decision-making (Sanfey, 2007). Knowledge of such factors can improve understanding of social-interactional processes and outcomes involving individuals, organizations and communities. Behavioral-genetic and evolutionary perspectives have been lacking from research in decision-making, with the exception of the few studies cited below. Behavioral-genetics (BG) is concerned with the genetic and environmental bases of phenotypic variation. BG offers models (e.g., twin and adoption designs) for examining sources of influence on measured behavioral traits (Segal, Chavar- ria, & Stohs, 2007). Evolutionary psychology (EP) is concerned with identifying physical and behavioral characteristics and functions promoting individual survival and reproduction (Buss, 2004). EP offers an informative theoretical framework for interpreting behavioral findings. Hamilton (1964) reasoned that altruistic acts (behaviors benefiting a recipient at some cost to the actor) should be directed more often toward close kin (i.e., individuals likely to carry shared alleles) than distant kin, as an indirect means by which one’s genes are preserved in future generations. However, proximal events underlying the expression and maintenance of closeness, cooperation and other beneficial interactions between relatives have been more speculative, although there are good candidates. The first study to include genetically informative players in an experimental game (Prisoner’s Dilemma) showed greater coopera- tion between monozygotic (MZ) co-twins than dizygotic (DZ) co- twins (Segal & Hershberger, 1999). More recently, Wallace, Cesarni, Lichtenstein, and Johannesson (2007) found that MZ twins make more similar rejection responses than DZ twins when interacting with anonymous players in an ultimatum game, demonstrating ge- netic influence. A pair of independently conducted twin studies using the trust game yielded the same results (Cesarini et al., 2008). However, both sets of investigators failed to pair co-twins with one another, forfeiting tests of evolutionary-based hypothe- ses concerning genetic relatedness, generosity and fairness. 1.1. Tacit coordination Tacit coordination refers to situations in which ‘‘two parties have identical interests and face the problem not of reconciling interests but only of coordinating their actions for their mutual benefit when communication is impossible” (Schelling, 1960, p. 54). Thus, tacit coordination may be conceptualized as non- negotiated consensus. Requirements for successful coordination include mutual recognition of signals that align each partner’s 0191-8869/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.paid.2008.06.022 * Corresponding author. Tel.: +1 714 278 2142; fax: +1 714 278 4843. E-mail address: [email protected] (N.L. Segal). Personality and Individual Differences 45 (2008) 607–612 Contents lists available at ScienceDirect Personality and Individual Differences journal homepage: www.elsevier.com/locate/paid

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Page 1: Tacit coordination in monozygotic twins, dizygotic twins and virtual twins: Effects and implications of genetic relatedness

Personality and Individual Differences 45 (2008) 607–612

Contents lists available at ScienceDirect

Personality and Individual Differences

journal homepage: www.elsevier .com/locate /paid

Tacit coordination in monozygotic twins, dizygotic twins and virtualtwins: Effects and implications of genetic relatedness

Nancy L. Segal a,*, Shirley A. McGuire b, Steven A. Miller a, June Havlena a

a California State University, Psychology Department, 800 N. State College Building, Fullerton, CA 92834, United Statesb University of San Francisco, Psychology Department, San Francisco, CA, United States

a r t i c l e i n f o

Article history:Received 30 May 2008Received in revised form 17 June 2008Accepted 23 June 2008Available online 3 August 2008

Keywords:TwinsVirtual twinsCoordinationCooperation

0191-8869/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.paid.2008.06.022

* Corresponding author. Tel.: +1 714 278 2142; faxE-mail address: [email protected] (N.L. Segal).

a b s t r a c t

Game theorists have long been interested in why some partners are more successful at achieving mutualgoals than others. The present study was conducted to determine if tacit coordination (non-negotiatedconsensus) varies as a function of genetic relatedness between social partners. The sample included7–13-year-old monozygotic (MZ) twin pairs (n = 53), dizygotic (DZ) twin pairs (n = 85) and virtual twin(VT) pairs (n = 42; same-age unrelated siblings). Co-twin agreement on questions completed under indi-vidual and coordination conditions was compared across the three groups. MZ twins showed significantlygreater overall agreement than DZ twins and VTs, using multiple criteria. The implications of these find-ings are considered with reference to behavioral-genetic and evolutionary theories of behavior.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Game theorists have a continuing interest in why some partnersare more likely to achieve mutual goals than others (Sugden &Zammarrón, 2006). There has been increased appreciation for thecomplexity of social environments in which most decisions aremade. Variables of interest include individuals’ emotional stateand sensitivity to the intentions of others. The recent efforts ofneuroscientists and economists have defined the neuroeconomicapproach, aimed at identifying brain mechanisms associated withvarious classes of decision-making (Sanfey, 2007). Knowledge ofsuch factors can improve understanding of social-interactionalprocesses and outcomes involving individuals, organizations andcommunities.

Behavioral-genetic and evolutionary perspectives have beenlacking from research in decision-making, with the exception ofthe few studies cited below. Behavioral-genetics (BG) is concernedwith the genetic and environmental bases of phenotypic variation.BG offers models (e.g., twin and adoption designs) for examiningsources of influence on measured behavioral traits (Segal, Chavar-ria, & Stohs, 2007). Evolutionary psychology (EP) is concerned withidentifying physical and behavioral characteristics and functionspromoting individual survival and reproduction (Buss, 2004). EPoffers an informative theoretical framework for interpretingbehavioral findings. Hamilton (1964) reasoned that altruistic acts(behaviors benefiting a recipient at some cost to the actor) should

ll rights reserved.

: +1 714 278 4843.

be directed more often toward close kin (i.e., individuals likely tocarry shared alleles) than distant kin, as an indirect means bywhich one’s genes are preserved in future generations. However,proximal events underlying the expression and maintenance ofcloseness, cooperation and other beneficial interactions betweenrelatives have been more speculative, although there are goodcandidates.

The first study to include genetically informative players in anexperimental game (Prisoner’s Dilemma) showed greater coopera-tion between monozygotic (MZ) co-twins than dizygotic (DZ) co-twins (Segal & Hershberger, 1999). More recently, Wallace, Cesarni,Lichtenstein, and Johannesson (2007) found that MZ twins makemore similar rejection responses than DZ twins when interactingwith anonymous players in an ultimatum game, demonstrating ge-netic influence. A pair of independently conducted twin studiesusing the trust game yielded the same results (Cesarini et al.,2008). However, both sets of investigators failed to pair co-twinswith one another, forfeiting tests of evolutionary-based hypothe-ses concerning genetic relatedness, generosity and fairness.

1.1. Tacit coordination

Tacit coordination refers to situations in which ‘‘two partieshave identical interests and face the problem not of reconcilinginterests but only of coordinating their actions for their mutualbenefit when communication is impossible” (Schelling, 1960,p. 54). Thus, tacit coordination may be conceptualized as non-negotiated consensus. Requirements for successful coordinationinclude mutual recognition of signals that align each partner’s

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608 N.L. Segal et al. / Personality and Individual Differences 45 (2008) 607–612

expectations with the other’s (Mehta, Starmer, & Sugden, 1994).Thus, the aim is not to predict what the partner would think ordo, but to identify a course of action upon which each partner’sexpectations converge. For example, if two people became sepa-rated, it would be advantageous for each to go to a place that bothprefer, not to a place each person knows the other would individ-ually prefer.

Identifying a common cue or focal point facilitates tacit coordi-nation between partners. This can occur via accident, imagination,reasoning or interpersonal knowledge. It has been shown that thesame solutions to certain problems are likely to be provided by amajority of individuals. For example, most people name ‘‘1” whenasked to choose a positive number. Such outcomes may be ex-plained by the salience of clues reflecting the background, experi-ences and/or circumstances of a particular group or population(Mehta et al., 1994; Schelling, 1960). A key point is that the focalpoint is the same for virtually all group members.

Clues rendering one decision or strategy more likely than anyother have been termed primary salience (Mehta et al., 1994). Incontrast, secondary salience refers to one’s choice of a strategybased on its primary salience for the partner. Mehta et al. (1994)defined a third variety called Schelling salience, in which partnersseek a selection rule that would lead to consensus or coordinationif both follow it. Schelling salience is the focus of the present twin-adoption study of tacit coordination. Specifically, the issue underinvestigation is whether genetic factors underlie decisions andstrategies resulting in desired ends. Based on behavioral-geneticand evolutionary-based views of phenotypic resemblance and kinrelations, respectively, the expectation is that coordination shouldvary linearly, and positively, with the proportion of genetic overlapbetween interactants.

1.2. Twin and adoption studies of social relatedness

It is reasonable to expect monozygotic (MZ) twins to showgreater success on tacit coordination tasks than dizygotic (DZ)twins or unrelated siblings. This expectation is based partly onMZ twins’ greater resemblance in mental abilities, information-processing strategies and temperamental dispositions (Plomin, De-Fries, McClearn, & McGuffin, 2001; Segal & Hershberger, 2005). DZtwins would be expected to show greater success on tacit coordi-nation tasks than unrelated siblings for the same reasons.

Evolutionary-based research supports a positive relationshipbetween social closeness and genetic relatedness (see Burnstein,Crandall, & Kitayama, 1994; Segal, Seghers, Marelich, Mechanic,& Castillo, 2007b). Consistent with these findings are twin studiesshowing greater cooperation, caring, and intimacy between MZ co-twins, relative to DZ co-twins (Danby & Thorpe, 2006; Foy, Vernon,& Jang, 2001; Loh & Elliot, 1998; Neyer, 2002a; 2002b; Segal, 2000;Von Bracken, 1934). Greater MZ than DZ twin closeness and famil-iarity were also observed among twins reared apart (Segal, Hersh-berger, & Arad, 2003). More positive social relations have also beenreported by twins and siblings, relative to half-siblings and step-siblings (Reiss, Neiderhauser, Hetherington, & Plomin, 2000; Segalet al., 2003; Segal, 2007). This work further predicts greater successon tacit coordination tasks by MZ twins, relative to DZ twins andunrelated siblings.

1.3. Successful coordination: the search for mechanisms

Mechanisms underlying the strength and intensity of MZ twins’social relationships are less well documented, although there aresome informative findings. Self-ratings of physical resemblancecorrelated significantly with social closeness for reared apart MZand DZ twins (rs = .33 and .29, respectively), and with feelings offamiliarity for reared apart MZ twins (r = .46). The more similar

treatment of MZ twins than DZ twins by family and friends has alsobeen linked to the nature of their within-pair interactions (Adel-man & Siemon, 1986).

Twin relations have also been examined with reference to co-twin resemblance in abilities, temperaments and interests (Koch,1966; Segal et al., 2003). However, few meaningful associationsemerged from such studies. It is possible that twins’ perceptionsof their behavioral and physical similarities, rather than their sim-ilarities in objectively measured traits, more importantly affecttheir relationship quality (Segal et al., 2003). Consistent with thisview is Leek and Smith’s (1991) finding that grandparents’ percep-tions of personality similarity to their children and grandchildrencorrelated positively with the value of help, both given andreceived.

Perceived similarity was also shown to significantly mediate therelationship between zygosity and twins’ perceived and compara-tive closeness to their nieces and nephews (Segal, Seghers, Mare-lich, Mechanic, & Castillo, 2007b).

Selected rearing and life history events appear to variouslyinfluence MZ and DZ twins’ social-interactional processes and out-comes. However, these factors lack the specificity needed to ex-plain MZ twins’ close, synchronized relationships. The search formechanisms underlying MZ twins’ close social relationshipsshould benefit from a twin-adoption study of tacit coordination.The findings should also further understanding of social related-ness in other pairs of individuals.

Most importantly, it is not suggested that MZ twins’ sharedbehaviors explain their social relationship qualities; it is proposedthat the outcomes of their shared behaviors (i.e., common, non-negotiated decisions) underlie, and enable, their close social rela-tions. In other words, tacit coordination may be a signal of geneticrelatedness that fosters cooperation. Close relatives should coordi-nate more than distant relatives and, consequently, perceive eachother to be more similar, leading to greater cooperation. Thus, suc-cessful coordination (mutual efforts that fulfill a common goal)may be a precondition for cooperation (behavior by an actor ben-efiting both actor and recipient) and altruism.

The foregoing would support the notion of genetically influ-enced focal points—features attended to by close relatives, but lessso by others. It is also expected that twin group differences in suc-cessful coordination (i.e., Schelling salience) will be more pro-nounced when judged using strict (exact match) criteria, ascompared with less strict (broad match) criteria. (Exact matchesindicate identical responses; broad matches indicate slightly var-ied responses; see below.) Narrowing definitions of consensus im-plies greater within-pair concordance in processing informationabout solutions to problems.

Behavioral-genetic and evolutionary psychological theories of-fer meaningful frameworks for interpreting the results. Theseseemingly disparate disciplines have been the recipients of in-creased joint attention in recent years (Keller & Miller, 2006). Asindicated above, BG offers an array of research designs for assess-ing evolutionary-based hypotheses concerning kinship and socialrelations (Segal, 1993). EP offers a theoretical backdrop for evaluat-ing social behaviors and trait-specific heritabilities.

1.4. The present study

A twin-adoption study of tacit coordination was conducted aspart of TAPS (Twins, Adoptees, Peers and Siblings) (Segal, McGuire,Havlena, Gill, & Hershberger, 2007a; McGuire, Segal, Whitlow, Gill,& Clausen, in press). TAPS is a study of behavioral adjustment andsocial relatedness that includes twins and virtual twins. Virtualtwins (VTs) are same-age unrelated siblings reared together sinceinfancy. VTs provide a direct estimate of shared environmentalinfluences, and are a better behavioral comparison with MZ and

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N.L. Segal et al. / Personality and Individual Differences 45 (2008) 607–612 609

DZ twins than ordinary adoptive siblings whose placement agesand residential histories vary.

2. Methods

2.1. Participant sample

Participants included 53 MZ twin pairs, 85 DZ twin pairs and 42VT twin pairs between 7.33 and 13.04 years of age. The twins wererecruited as part of TAPS, a joint collaboration between CaliforniaState University, Fullerton and the University of San Francisco.The zygosity of the same-sex pairs was established by comparativeexamination of 13 short tandem repeat (STR) DNA markers. Male-female twin pairs (n = 33) were assigned as DZ because of their sexdifference.

The VTs were part of the ongoing Fullerton Virtual Twin Study(FVTS) of behavioral development. VT pairs are defined by the fol-lowing rules: (1) adoption by one year of age; (2) sibling age differ-ence of nine months, or less; (3) sibling enrollment in the sameschool grade; 4. absence of birth difficulties; and (5) current ageof four years, or older (see Segal 1997; 2000; 2006. Approximately120 VT pairs have been tested thus far, although only 43 pairs par-ticipated in TAPS, given the targeted age range of middle to latechildhood.

Participant characteristics are summarized in Table 1. The meanage of the participants was 9.98 years, SD = 1.45. MZ and DZ twinsdid not differ in age, but both groups were approximately ninemonths younger than the VTs, [MZ vs. VT: [t (93) = �2.96,p < .01]; DZ vs. VT: [t (125) = �3.40, p < .001]. The mean age differ-ence between VT co-twins was only 0.25 year (SD = 0.24, range:0.00 to 0.75 year). Individual participants were mostly Caucasian(68%), with the remainder including participants who were His-panic (6%), Asian (5%), Black (3%), Mixed (16%), and North Ameri-can Indian, South American Indian or Other (2%). Co-twins in 31VT pairs (74%) were of the same ethnicity.

2.2. Research design and procedures

Home visits lasting three hours were made to families of MZtwins, DZ twins and VTs by two TAPS examiners. Examinersadministered a battery of cognitive, personality and behavioraltasks to the children and to one parent, usually the mother. Unlesstesting activities involved joint behavior, each twin was assessedindependently, at the same time, by a different examiner.

The assessment included a two-part Social Coordination Ques-tionnaire. This form was modeled after an inventory by Mehta etal. (1994), with modifications to increase its appropriateness forchildren. Part I included 10 questions that children were instructed

Table 1Age and gender of MZ twins, DZ twins and virtual twins

Twin group n (pairs) Age (in Years)a,b Gender

(SD) Range M F OS

MZ 53 9.77 19 34 –(1.43)8.00–12.92

DZ 85 9.78 32 20 34(1.32)

8.00–12.75VT 42 10.67 8 7 27

(1.53)7.33–13.04

Members of several triplet and quadruplet sets were entered as pairs.M = Male; F = Female; OS = Opposite-sex.

a MZ > VT, p < .01.b DZ > VT, p < .001.

to ‘‘answer any way that you like” (Individual Condition). Forexample, they were asked to ‘‘name a flower,” and ‘‘write any boy’sname.” Upon completion, the children answered the questionsagain, but were instructed to respond so that they would ‘‘endup with the same answers” as their twin (Coordination Condition).

This second trial was followed by interviews concerning familyand friends. A new set of ten questions (Part II) was then intro-duced with the same instructions for answering them under theIndividual and Coordination conditions.

Responses were scored by USF staff members who were blind tothe twins’ zygosity. Three judges had to agree on the nature of thematch. Eight scores, one for each of the four test conditions (Indi-vidual-Part I, Individual-Part II, Coordination-Part I, Coordination-Part II), using two scoring schemes (broad match and exact match),were generated. Scores ranged from 0 to 12 for Part I (one item in-cluded three parts); scores ranged from 0 to 10 for Part II. If co-twins answered ‘‘Ron” and ‘‘Ronald” when asked to write a boy’sname, or ‘‘red” and maroon” when asked to name a color, the pairreceived a broad match point. If both twins answered ‘‘Ron,” orboth twins answered ‘‘red,” the pair received an exact match point.The two coding schemes allowed assessment of response evalua-tion consistency. Interrater reliabilities for a subsample of 35 pairsranged between .87 and .99.

3. Results

3.1. Correlations between broad match and exact match scores

Correlations between broad match and exact match scores(n = 179–180) were high and significant at p < .001: individual, PartI: r = .97; coordination, Part I: r = .97; individual, Part II: r = .96;coordination, Part II: r = .96. Thus, evaluation of responses was con-sistent across the two scoring schemes.

3.2. Correlations between pair age, pair gender, pair ethnicity andsocial coordination scores

Correlations between age and social coordination scores werenegligible except for a significant, but modest, correlation betweenage and the Part II coordination scores (broad match: r = .31,p < .001, n = 179; exact match: r = .31, p < .001, n = 179). Pair gen-der (same-sex male, same sex-female or opposite-sex) and VT pairethnicity (same or different) were uncorrelated with the socialcoordination scores.

3.3. MANOVA: social coordination scores by zygosity groupmembership

The data were analyzed by repeated measures MANOVA (Gen-eral Linear Model), with one between-group factor (Zygosity: MZ,DZ, VT) and two within-subjects factors (Condition: Individual,Coordination) and (Questions: Part I, Part II). The analyses wereconducted twice, using both the broad match and exact match cri-teria. A larger effect size was associated with the exact match(stricter) criteria, as expected, although the pattern of findingswas identical. Therefore, only exact match analyses are presented;mean scores summarized in Table 2.

3.4. Exact match

A significant multivariate effect of Zygosity emerged [F (2,176) = 7.73, p < .001, g2

P ¼ :08]. MZ twins obtained the highestoverall score, as well as the highest scores across all four measures.Both condition [F (1,176) = 101.33, p < .001, g2

P ¼ :36] and ques-tions [F (1,176) = 9.45, p < .01, g2

P ¼ :05] showed significant multi-

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Table 2Mean social coordination scores for MZ twins, DZ twins and virtual twins

Exact match criteria

Individual – Part ITwin group Mean SD RangeMZ 2.38 1.50 0–6DZ 2.02 1.26 0–5VT 1.50 1.29 0–5Total 2.01 1.37 0–6

Individual – Part IIMZ 2.02 1.21 0–4DZ 1.55 1.38 0–7VT 1.29 1.22 0–5Total 1.63 1.32 0–7

Coordination – Part IMZ 3.15 1.80 0–7DZ 2.93 1.63 0–7VT 2.38 1.65 0–6Total 2.87 1.70 0–7

Coordination – Part IIMZ 2.88 1.72 0–6DZ 2.24 1.45 0–6VT 2.21 1.32 0–5Total 2.42 1.53 0–6

n (pairs): MZ = 52–53; DZ = 85; VT = 42.

610 N.L. Segal et al. / Personality and Individual Differences 45 (2008) 607–612

variate effects. Participants scored higher on items answered in thecoordination condition than in the individual condition, and scoredhigher on Part I questions than on Part II questions. MZ twinsscored higher than the other two groups on both the individualand coordination questions as expected, but none of the two-wayor three-way interactions involving Zygosity, Condition and/orQuestions were significant.

The linear contrast was statistically significant [contrast esti-mate = �5.45, SE = .14, 95% interval: �.82 to �.27, F(1,176) = 15.15, p < .001]. The quadratic contrast was not signifi-cant, as expected [contrast estimate=.04, SE = .12, 95% interval:�.19 to .27, F (1,176) = 0.10, NS].

Interactions between zygosity and gender (male–male, female–female) were examined using MZ twins, same-sex DZ twins andsame-sex VT twins. Zygosity remained significant [F (2,113) =5.55, p < .01, g2

P ¼ :09], but the zygosity � gender interaction wasnon-significant [F (2,113) = 0.68]. A second analysis, using onlyDZ twins and VT twins, yielded a significant zygosity effect [F(1,121) = 4.07, p < .05, g2

P ¼ :03, but a non-significant zygos-ity � gender (male–male, female–female, male–female) interac-tion [F (2,121) = .00].

4. Discussion

4.1. Tacit coordination and social relatedness

Findings from the first twin-adoption study of tacit coordina-tion demonstrated greater success by MZ twin pairs, followed byDZ and then VT pairs. A significant linear contrast showed the pre-dicted positive relationship between genetic relatedness and coor-dination. These results are consistent with the extant behavioral-genetic literature showing that cognitive and personality resem-blance varies as a function of genetic relatedness between familymembers. The results are also consistent with evolutionary psy-chological predictions, namely that greater alignment of effortsand goals is expected between close genetic relatives than betweendistant genetic relatives. Most importantly, the present study of-fers a new perspective on proximal events that may facilitate coor-dination, as well as closeness, cooperation and other positive socialexchanges between relatives that have heretofore been morespeculative.

4.2. Proposed mechanisms underlying social relatedness

Proposed mechanisms mediating relationships between geneticrelatedness and altruistic/cooperative behaviors include emotionalcloseness (Korchmaros & Kenny, 2001), feelings of familiarity(Neyer & Lang, 2003) and perceptions of phenotypic resemblance(Segal et al., 2007b; Tancredy & Fraley, 2006; Trivers, 1985). Differ-ent types of recognition have also been associated with differenttypes of emotions and biochemical states ‘‘which in turn triggera cascade of physiological and psychological events affecting bothsignaling and recognition” (McGuire, Fawzy, Spar, Weigel, & Troisi,1994). However, these explanations leave open the question of ex-actly what is recognized.

Tacit coordination (and mutual awareness of non-negotiatedmatched choices) may represent a class of mechanisms by whichclose relatives develop and maintain close social relations. Com-mon, independently made choices, ranging from the mundane(e.g., purchasing similar outfits) to the meaningful (e.g., enteringthe same occupation), may function as unambiguous indicatorsof similarity. Active gene-environment correlation (the selectionor modification of experience, based upon individuals’ geneticallyinfluenced proclivities) may underlie the congruence of MZ twins’choices (Plomin et al., 2001), and the greater congruence of DZtwins’ choices relative to those of VT twins. The clarity of theseindicators distinguishes them from twins’ measured or perceivedsimilarities in intelligence and personality that may be more globaland, therefore, less apparent to the various partners. The fact thatthe twin group differences were maintained across both the indi-vidual and coordination conditions also deserves emphasis. Thissuggests that more closely related individuals show more closelymatched thought processes regardless of whether coordination isconsciously trying to be achieved.

Clear indicators of similarity (i.e., those associated with tacitcoordination), once recognized, may solidify social bonds byaffirming similarity. This might be especially true in situationsrequiring unusual or atypical non-negotiated actions leading tomutually desired goals.

In summary, the present study suggests a genetic basis forattention to selected focal points, the characteristics or clues pre-disposing individuals to favor one choice or strategy over another.As such, this behavior may be conceptualized as an outcome orconsequence of one’s genetic background, in conjunction with lifeexperiences. Many situations, including the Social Coordinationquestions completed by the twins, offer a range of decisions. With-in-pair congruence for important, tacitly agreed upon choices mayserve as unambiguous indicators of similarity, providing a basis forcooperative behavior. This same process may partly explain theeither close (or distant) social relations between other pairs of rel-atives and individuals.

It might be objected that the greater success of the MZ twinscould be explained by the likelihood that they shared more timewith one another than did the DZ twins and VT twins. However,even if MZ twins did spend more time together it could be arguedthat their genetic relatedness and perceptions of similarity drovetheir relationship, rather than the other way around. Researchshows that MZ twins who are treated alike are not more behavior-ally alike than those treated differently (Segal, 2000). In addition,reared apart twins’ contact time prior to assessment showed neg-ligible correlations with their initial and current closeness andfamiliarity (Segal et al., 2003).

4.3. Future directions

Biological and adoptive parent-child data would allow furthertests of the observed associations. Subsequent research wouldalso benefit from analyses of responses to specific inventory

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N.L. Segal et al. / Personality and Individual Differences 45 (2008) 607–612 611

items. The present study utilized summary scores, but someitems may have had primary salience for the group or popula-tion of which the twins were members. Eliminating such itemsmay have increased the twin group differences. Information con-cerning specific cues or focal points used in the coordinationcondition might be obtained by debriefing twins following theirparticipation; one approach to this question would involvereared apart twins who have had little contact. Finally, while itis likely that increased coordination predicts increased coopera-tive and altruistic behavior, the addition of activities to assessthis association would be of interest. These activities might offerprizes for successful coordination/cooperation, given that incen-tives can affect performance in experimental games (Parks,2000).

4.4. Other implications

Results from the present study have implications for behavior inother contexts. What might appear to be ‘‘telepathic communica-tion” between twins (or other individuals) may be resolved withreference to the present findings. An alternative conceptualizationof this phenomenon may be that MZ twins’ capacities for tacitcoordination give rise to telepathic-like behaviors (e.g., choosinga vacation spot that both would enjoy). In a paper exploring friend-ship and the evolution of altruism, Tooby and Cosmides (1996) ask,‘‘Who is good at reading your mind?” (p. 137). Their answer in-cludes dyads who communicate effectively and who intuitivelyunderstand each other’s ideas and objectives; in short, individualswith capacities for tacit coordination.

The foregoing raises the possibility of a ‘‘twin mechanism” facil-itating discrimination between MZ and DZ co-twins. However, thatseems unlikely, given the relative infrequency and increased haz-ards of twinning in human populations. MZ twins’ greater with-in-pair affiliation would seem to be an extension of the moregeneral principles of kin recognition and inclusive fitness.

Tacit coordination would seem to be a significant psychologicalmechanism, affecting outcomes in many situations. It may increasethe probability of positive outcomes in various social, industrial,economic and military contexts that depend upon common solu-tions. Schelling (1960) acknowledged this, as well as the fact thattacit coordination is affected by multiple factors. He did not, how-ever, consider the possibility of genetically influenced focal points.Admitting this view into current classes of explanation may assistour understanding of the success of family businesses, or the jointpolitical ascent of Poland’s MZ twin president and former primeminister.

A genetically informed view of tacit coordination also brings anew perspective to processes facilitating successful social ex-change (i.e., communication of one’s values, desires and needs,and recognition of them in others), as specified by social contracttheory (Buss, 2004; Cosmides & Tooby, 1992). Twins and sib-lings, by variously sharing, communicating and recognizing eachother’s social circumstances, may provide models for studyingthe development of social exchange in experimental and naturalsettings. They may help explain the existence of unexpected liai-sons and unlikely coalitions (e.g., negotiations between feudingfamilies; alliances between political opponents). The key pointis that rewards are most likely when social niches are filled withindividuals sharing goals and interests (Tooby & Cosmides,1996).

Examining the nature and significance of focal points has beenidentified as a research area with rich, but unrealized, potentialfor understanding consensus and cooperation (Sugden & Zam-marrón, 2006). Creative designs using genetically, socially and cul-turally informative participants should enhance understanding ofcommunication and negotiation across time and place.

Acknowledgements

This study was supported by NIMH Grant R01 MH63351. JohnQ. Patton provided critical commentary for earlier versions of themanuscript.

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