cooperation and competition as a function of zygosity in 7- to 9-year-old twins

Evolution and Human Behavior 19: 397–411 (1998)

1998 Elsevier Science Inc. All rights reserved.655 Avenue of the Americas, New York, NY 10010

1090-5138/98/$19.00PII S1090-5138(98)00034-8

Cooperation and Competition as a Function of Zygosity in 7- to 9-Year-Old Twins

C.Y. Loh and John M. Elliott

Department of Social Work and Psychology, National University of Singapore

A study of 31 monozygotic (MZ) and 24 dizygotic (DZ) Singapore twin pairs (aged 7.6 to9.7 years) is reported. MZ and DZ twins differed in cooperative and competitive behav-iors measured by within-pair reactions to payoff structures designed to elicit simulta-neous cooperation (SCO) or reciprocal altruism (RA) on a marble-pull task. Reciproca-tion entailed turn-taking in rewards. DZ twins showed greater competition than MZtwins when competing for marbles in the RA condition, as predicted. Unexpectedly,however, MZ twins were markedly more competitive than DZ twins under the SCO con-dition, when success on the task always rewarded both twins. The competition aroseover which end of the apparatus was to be used on a given trial. This interaction betweencondition and zygosity could not be related to differences in parents’ reported treatmentof their children or zygosity beliefs, nor did it appear to be related to differences in fac-tors such as time spent together and shared activities. An explanation in terms of uncer-tain or unstable dominance in MZ twinships is suggested. © 1998 Elsevier Science Inc.

KEY WORDS:

Children; Competition; Cooperation; Dominance; Reciprocal Altruism; Sin-

gapore; Twins; Zygosity.

nclusive fitness theory predicts that the greater the genetic relatedness be-tween people, the higher the chance that they will act altruistically. Reciprocalaltruism or turn-taking is an example of cooperation, and Segal (1984) sug-gested that we might expect monozygotic (MZ) twins to be more cooperative

towards each other than dizygotic (DZ) co-twins. This might occur if, for example,the psychological mechanisms mediating altruism are sensitive to perceived similar-ity. Segal (1988) comprehensively reviewed the evidence and argument for her con-clusion, and her own results supported it (Segal 1984).

I

Received April 30, 1998; revised August 31, 1998.

Address reprint requests and correspondence to: John M. Elliott, Department of Social Work and Psy-chology, National University of Singapore, Singapore 119620.

398

C. Y. Loh and J. M. Elliott

The present article presents an essentially serendipitous finding suggesting that MZtwins are in certain circumstances more competitive than DZ twins. We used a gametask in which the reward structure could be varied to allow reciprocal altruism orsimultaneous cooperation over a series of trials. DZ twins were more competitive inthe former case, where only one member of a twin dyad could win on a trial. MZ twinswere more competitive in the latter, when both members of the dyad benefitted onany given trial. This suggests that the nature of “cooperation” has to be specifiedprecisely when discussing the effects of genetic similarity on cooperative behavior.

COOPERATION AND RECIPROCAL ALTRUISMVERSUS COMPETITION

The word “cooperation” generally refers to cooperative behaviors in contexts whereindividuals cannot achieve alone what cooperation makes possible. Cooperation caninclude reciprocal altruism and simultaneous cooperation, among others (Rothsteinand Pierotti 1988). For example, Knight et al. (1985) describe three distributionrules in children’s cooperative decision-making: maximizing the gain of another(altruism), minimizing the difference between own and other’s gains (equity), andmaximizing joint gains (group enhancement). Similar distribution rules have beenused by social psychologists investigating in-group bias (Tajfel 1982).

Rothstein and Pierotti (1988), however, stress the need to distinguish betweenaltruistic and other cooperative behaviors, especially simultaneous cooperation. Inreciprocal altruism, whereas altruist and beneficiary both experience net benefits inthe long run, they must experience unequal costs and benefits on particular occa-sions. In simultaneous cooperation, such a difference does not occur. Cooperationfor shared benefit can be renegotiated at any time, but there is no unreciprocated ob-ligation arising from a particular transaction. Reciprocation, on the other hand, canonly occur on a task with a reiterated element, implying some stability of relation-ship across time. Both reciprocal altruism and simultaneous cooperation are suscep-tible to disruption by competitive behaviors that serve to maximize individual gainsat the expense of others.

Segal (1984) compared MZ and DZ 8-year-old twin pairs. In her study, “coop-eration” was a situation in which each twin worked at the task, but points accumu-lated were awarded to the co-twin. “Competition” was the situation when a twinscored points for himself or herself. A twin did not know what the co-twin waslikely to do in either situation.

On the differential productivity task reported by Segal (1984), which was basedon Madsen (1967), MZ twins worked harder for their co-twins than DZ twins did fortheirs. A jigsaw puzzle completion task yielded similar results: MZ twins consistentlycooperated to solve the puzzle while DZ twins often worked separately and showedgreater role differentiation in terms of dominance and submission. Segal interpretedthese findings as showing greater restraint of selfishness on the part of the MZ twins.

Although these results were as predicted, it is not certain whether Segal wasmeasuring reciprocal altruism or some other kind of cooperation. Segal likened her

Cooperation and Competition in Twins

399

game structure to the prisoner’s dilemma (PD) of Axelrod and Hamilton (1981), butthe latter entailed reiterated decision-making with consequent opportunities forcheating or retaliation. In Segal’s task, there were no such opportunities, and it was,at best, only like a single PD situation. Although a single PD does create a dilemma,because there is no way of telling in advance if cooperation will occur, only reitera-tions allow opportunities for reading the partner’s strategy. Long-term interactionbetween partners allows reciprocal altruism (turn-taking) to emerge.

Whereas reiteration renders the PD a model for reciprocal altruism, it is never amodel for simultaneous cooperation. Nor is the notion of cooperation of Segal(1984) equivalent to the criteria for simultaneous cooperation of Rothstein andPierotti (1988). “Cooperation” in the differential productivity task does not involvethe necessity of combined action. The decision not to cooperate does not affect thedecision-maker’s reward; the combined payoff is not necessarily more than twicethe payoff for one individual; and cheating is not costly to the party that cheats. Infact, Segal used the terms cooperation and altruism interchangeably in describingthe behavior measured by the differential productivity task. The study reported hereaddresses the same issue but uses a social psychological task (using a marble-pullapparatus) that measures cooperation and competition in payoff structures that re-flect sociobiological definitions of cooperation and reciprocal altruism.

SOCIAL BEHAVIORS OF TWINS

The life experiences of twins differ from those of singletons in significant ways, e.g.,not knowing physical loneliness in childhood and always sharing parents’ attention(Rutter and Redshaw 1991; Segal 1988). At an early age (20 months) there are dif-ferences in the responsiveness of MZ and DZ twins to same-aged distressed victims(Zahn-Waxler et al. 1992), although zygosity in older (8–11 years) children does notpredict cooperative behavior with similar-aged strangers (Segal et al. 1996). Towhat extent such differences reflect variations in social experience is uncertain.

Among twins themselves, MZ twins differ from DZ twins in that the formershare greater intimacy than the latter (Segal 1984, 1988; Siemon 1980; Smith et al.1968). Von Bracken (1934) demonstrated that young MZ twins tended to maintainequality during mathematical tasks, even slowing down performance to enable co-twins to catch up, whereas DZ twins tended to engage in competition with co-twins.Von Bracken concluded that MZ twins showed a high degree of attachment withstrong mutual liking and a tendency towards coordination. On the other hand, DZtwins showed considerably less attachment with less intense mutual liking and a ten-dency for each co-twin to feel superior.

Comparison of MZ and DZ twins in this, as in other, fields inevitably raises aquestion regarding confounding of genetic effects and the effects of nonshared en-vironments. There is, however, good reason to suppose that if MZ and DZ twins dif-fer in cooperative and competitive tendencies, and even if these differences resultfrom nonshared environments, they originate in the actual genetic similarity of thetwins.

400


Patterns of association between parental and childhood behaviors are differentfor MZ and DZ twins (Dibble and Cohen 1980), and parental perceptions of differ-ences in twins during infancy also have lasting effects on the social and cognitivedevelopment of the twins (Hay and O’Brien 1984). However, caretakers may re-spond to the physical similarities of MZ twins by encouraging or creating more sim-ilar environments (Rowe 1981; Scarr and Carter-Saltzman 1979). In the study byScarr and Carter-Saltzman (1979), similarities and differences between twins werefound to be predicted by the twins’ true zygosity rather than mistaken zygosity. Per-ceived similarity was not found to be an important bias in the study of genetic vari-ance in intellectual skills. For personality variables, DZ twins who believe them-selves to be MZ may be influenced by perceived zygosity. However, Scarr andCarter-Saltzman (1979) reported that the blood group data indicated that DZ twinswho had fewer genetic differences were the ones who believed themselves to be MZtwins. The study concluded that, although MZ twins experience more similar envi-ronments in general, this appears to be a result of their actual genetic similarities.

Cohen et al. (1977a, 1977b) applied the same argument in a study that con-cluded that parental expectations of zygosity did not appear to strongly influencesimilarity in twin children. MZ twins were found to be more similar than DZ twinson dimensions of attention, behavior modulation (or activity), and sociability. MZtwins whose parents were mistaken or unsure about their children’s zygosity were asalike in these respects as MZ twins believed by parents to be MZ twins. Lytton(1977, 1980) similarly concluded that parents of twins respond to, rather than create,differences between their twin children. Such studies suggest that environment doesnot explain phenotypic similarities independently of zygosity.

THE PRESENT STUDY

The present study re-examined the hypothesis that MZ twins generally are morecooperative than DZ twins. The main independent variable was zygosity. Thehypothesis was tested in two different conditions, simultaneous cooperation (SCO)and reciprocal altruism (RA), so labeled because of their potential to elicit thesetypes of cooperative behavior, respectively. The dependent variables were the out-come behaviors of the twins under these two conditions. Our primary purpose wasto establish whether zygosity differences occurred in conditions favoring coopera-tive and reciprocating behaviors, respectively.

METHOD

Subject Sampling

Subjects were restricted to the age range 7 to 9 years, as in the study by Segal(1984). This also avoided age-related changes in cooperative and competitivebehaviors, noted by Li et al. (1979) using a marble-pull apparatus with Chinese chil-dren from Hong Kong and Taiwan. For DZ twins, the same sex was specified to


401

avoid the likely additional effect of within-pair sex differences (Rushton et al. 1986;Segal et al. 1996).

Singapore is an island nation of about 2.9 million people (MITA 1995). A na-tionwide appeal to primary schools resulted in a sample of 89 same-sex twin pairswho met these preliminary requirements, were willing to participate, and whose par-ents and principals consented to their inclusion.

Fuller details of sampling and zygosity determinations are given in Loh (1995)and Loh and Elliott (1996). We wanted to avoid invasive procedures, such as bloodtyping, and utilized a structured interview based on Goldsmith’s Zygosity Question-naire for Young Twins (Goldsmith 1991). We adapted it to allow for face-to-face ortelephone vernacular or English interviews with parents. Careful attention was givento consistent small differences in hair and eye color, because Asians generally haveblack hair and brown eyes.

Although all Singapore children attend English medium schools, English usu-ally is not their native language. Competence in English could differ between co-twins, leading to a different understanding of game instructions. It also is desirablethat within-twin pair differences in reasoning skills and general competence areminimal in any twin study, where these differences may affect performance. Forexample, Segal (1984) checked concordance for IQ in her sample of MZ and DZtwins. Accordingly, all our pairs were assessed for reasoning and language abilitiesusing the British Abilities Scale-Matrices (BAS-M) (Elliott 1983) and The BritishPicture Vocabulary Scale (BPVS) (Dunn et al. 1982). These tests were selected be-cause provisional local norms were available (Rickard-Liow and Tng 1992a,1992b). Pairs who showed differences of more than 5 points for BAS-M or 8 pointsfor BPVS were eliminated from the subject pool. These values were close to 1

SD

for BAS-M and BPVS in this sample (4.8 and 6.8, respectively).Twenty-three pairs were eliminated. It is possible that restricting the study to

homogeneous pairs affected the range of within-pair competitive or cooperativebehaviors found. In the event, however, clear zygosity differences that required anexplanation emerged nevertheless. An additional 11 pairs were excluded for reasonsunrelated to the study design (e.g., interruptions by well-meaning teachers whocould not resist telling the children the “best strategy” to use). The final samplecomprised 31 MZ and 24 DZ twin pairs, for a total of 55 pairs (Table 1).

The sex and ethnic characteristics of the sample are given in Table 2. The MZgroup was less representative of the ethnic variation in Singapore than the DZgroup. MZ twins are predominant in both Malay and Chinese families (Foong

Table 1. Mean (

SD

) Age, BAS-M and BPVS Scores for 31 MZ and 24 DZ Twin Pairs

MZ (

n

5

31) DZ (

n

5

24)

t

(1, 53)

Age (years) 8.54 (0.59) 8.71 (0.64) 1.02* BAS-M 14.24 (4.91) 16.04 (4.52) 1.40*BPVS 33.87 (5.91) 35.15 (7.90) 0.69*

*

p

5

NS.

402


1970), but Malay parents seemed more willing to allow their children to participatein the study.

Apparatus

The apparatus was based on that developed by Madsen and Lancy (1981). It con-sisted of a closed rectangular plastic box (43 cm high, 15 cm wide, and 62 cm long)with an eyelet screwed to the top surface at each end. Strings ran through the eyeletand connected to a plexiglas marble-holder placed centrally on the top. Glass mar-bles were used as rewards. The marble holder could be pulled towards either player(seated at each end of the box), to deliver the marble into a cup (hole) located nearthe player. These target cups were labeled “sun” and “moon”, respectively. Themarble holder was held together by magnetic inserts that parted if players pulledagainst each other. When that happened, the marble did not enter either target cupbut rolled into drains on either side of the box. A round was terminated when eitherthe marble was delivered into a cup or the marble holder parted.

The apparatus delivered the marbles via tubes running from each of the targetcups to collection boxes accessible to each twin. The top surface of the box and thetube system were transparent, and the children could see the marbles rolling throughthe tubes into their respective collection boxes. Marbles that did not fall into the tar-gets were returned to a pool. The children could play as many rounds as it took towin all the marbles.

Design

Independent variables were the degree of genetic relatedness (MZ or DZ) and game(payoff) conditions. There were two different payoff structures, reflecting SCO andRA, respectively.

Condition SCO was always played first. Both participants were rewarded withone marble on every successful trial. Twenty marbles were allocated as rewards, 10for each twin obtainable in a minimum of 10 rounds. Cheating was impossible, asthere was no procedure whereby any subject could gain a relative advantage in re-wards either within or between rounds. Competition could disrupt cooperation. Ifthe participants could not agree whose cup was to be used and the holder broke, both

Table 2. MZ and DZ Twin Pairs in the Final Sample Grouped by Sex and Ethnic Group

MZ (

n

5

31) DZ (

n

5

24)

M F M F Total % National %

Malay 7 9 4 2 22 40 14Indian 2 0 1 2 5 9 7Chinese 8 5 7 8 28 51 78Others 0 0 0 0 0 0 1Totals 17 14 12 12 55 100 100


403

parties went unrewarded on that round. Such failure could extend to any number ofrounds.

In condition RA, the payoff structure allowed (but did not compel) reciprocalaltruism. Ten marbles were allocated as rewards. On any given round, the marblewent only to the twin into whose cup it was maneuvered. It was thus open to thedyad to maintain a reciprocal pattern of turn-taking and thus equalize rewards overrounds, but a nonaltruistic pattern also was possible, in which one twin dominatedand successfully persuaded or coerced the other into a lopsided reward allocation.As with SCO, a failure to agree could disrupt the game over an indefinite number ofrounds.

Equal distribution of rewards between co-twins need not indicate a high degreeof cooperation. It depends on how many times the marble holder was broken in theprocess. A score of 10:0 (one twin gets all) indicates submission, but the number ofbreaks indicates how easily this submission took place (Sparkes 1991). Thus, fol-lowing Sparkes, we used two dependent measures. The first is the number of roundsplayed, taken as a measure of competition. It is measured in both the SCO and RAconditions. Minimum rounds played indicate cooperation. The second is the ratio ofcups selected (twin A:twin B), taken as an estimator of relative dominance. Thismeasure corresponds to the ratio of rewards in RA. In SCO, as rewards are alwaysequal, it just assesses the extent to which there is inequality with respect to the cupactually used.

Procedure

Twins were seen in their respective schools by the first author. She explained thatthis was a game and that they could win marbles to take home. The twins wereseated one at each end of the apparatus.

Condition SCO was played first. The experimenter took 20 marbles and ex-plained that, in the first game, twins would both win a marble whenever one fell intoeither cup. She demonstrated this by delivering a marble into a cup via the appara-tus, while taking another marble from the pool of 20 and dropping it into the emptycup. She emphasized that it did not matter which cup the marble went into, bothplayers would be rewarded and demonstrated this principle again using the othercup. The twins were told they would be playing to win all 20 marbles and remindedthem they could keep marbles that they won. All marbles won were to be displayedduring the game so that everyone could see the number won by each child. Theyalso were told they could talk to each other about what they wanted to do.

The marble holder was set up at the center of the apparatus, and a marble from thepool of 20 marbles was placed in the marble holder. The experimenter said, “Ready?Go,” at the start of every round. If the marble holder broke apart and the marblewithin rolled off, she returned it to the pool. If the marble fell into either cup, shetook another marble from the pool and dropped that into the empty cup, saying “onemarble for each of you!”. Trials continued until all 20 marbles in the pool were won.

At the end of the game, twins were asked to report the number of marbles wonand told to keep them in their pockets. The number won was always the same, and

404


the purpose of the report was to highlight a change in game rules as the experimentmoved on to the RA condition.

For condition RA, the experimenter reminded the twins that there were still 10marbles, to be won in a second game. She stressed that the rules of the second gamewere different and suggested the cups be called A’s cup and B’s cup, respectively.She explained that, in this new game, if a marble fell into a cup, only the personwhose cup it was won a marble. This rule was demonstrated for each cup in turn.

When one marble fell into a cup, the experimenter did not add another marbleinto the empty cup, but just remarked “one marble for. . . .(one twin), no marblefor. . . .(the other twin).” Trials continued until all 10 marbles were won. All otheraspects of the procedure were as in the SCO condition.

In the few cases where a game reached the 150th round, the experimenterwould ask the twins to discuss a plan of action before the next round. Thereafter, af-ter every 10 rounds, the children were asked to discuss their plan before continuing.

RESULTS

Numbers of Rounds Played

Mean rounds played (minimum 10) are given in Table 3, and rounds for each twinpair are shown graphically in Figure 1. The data are graphed separately for Chineseand non-Chinese MZ and DZ twin pairs using common axes. The Indians are asmall minority (

n

5

5) and were grouped with the Malays, whose results they mostclosely matched. The data are clearly skewed, and, as the sample sizes are alsounequal, each pair score was converted to log10 values for analysis. This reducedskew values below 1.00.

Analysis of variance showed no main effect of zygosity [

F

(1, 51)

,

1], ethnicgroup [

F

(1, 51)

5

3.75,

p

5

0.06], or conditions [

F

(1, 51)

5

0.31,

p

5

0.09],although the latter effects approached significance. The interest lies in the stronginteractions. Overwhelmingly, the variance was accounted for by the interactionbetween zygosity and condition [

F

(1, 51)

5

117.3,

p

,

0.001]. MZ twins took signif-icantly more rounds in condition SCO and DZ twins significantly more in RA. Thisreversal of the pattern of scores across conditions for MZ versus DZ pairs explainsthe lack of any main conditions effect. There were small but significant interactions

Table 3. Mean Rounds (

SD

) Played by Zygosity, Condition, and Ethnic Group

MZ DZ

SCO RA SCO RA

Chinese 49.9 15.0 17.9 28.3(45.0) (4.95) (9.32) (21.6)

Non-Chinese 48.7 16.0 17.3 53.0(34.5) (4.60) (3.61) (23.2)

All 49.2 15.6 17.7 37.6(38.5) (4.70) (7.58) (24.9)

Note

. Higher values imply that pairs were slower to cooperate. A minimum of 10 rounds was possible with perfect coop-eration.


405

between ethnicity and condition [

F

(1, 51)

5

5.52,

p

,

.05], and zygosity, ethnicity,and condition [

F

(1, 51)

5

4.89,

p

,

.05], reflecting the tendency for Chinese DZtwins to show a less marked reversal of pattern across conditions.

Figure 1 clearly shows the absence of any ethnicity effect for MZ twins. Withonly one exception in the entire set of 31 pairs studied, the number of rounds neededto complete the task was greater for SCO than for RA, sometimes very muchgreater. For the DZ pairs, Figure 1 shows a small effect of ethnicity, but an over-whelming effect of zygosity. The difference between conditions is in the oppositedirection from MZ twins. The number of rounds needed usually was greater for RAthan for SCO. However, this effect was significantly less for Chinese twins.

Sex of dyad was never significant as a main effect or in any interaction [

F

(1,51)

,

1.3 in all analyses].

Marble-Position Ratios

A “marble-position” (MP) ratio of the numbers of marbles that fell into the twocups was used to categorize twins for analysis. A balanced MP ratio would indicate

FIGURE 1. Total rounds played in simultaneous cooperation (SCO) and reciprocal altruism(RA) conditions. Each line joins the scores for a single twin pair under each condition. a) MZChinese pairs; b) MZ Malay and Indian pairs; c) DZ Chinese pairs; d) DZ Malay and Indianpairs.

406


equality in dominance. Difficulties in arriving at this balance indicate unresolveddominance, whereas a swift resolution indicates preestablished equality in domi-nance. Using MP ratios and number of rounds played, MZ and DZ dyads were clas-sified into the categories of cooperative and competitive behaviors outlined bySparkes (1991) (Table 4).

MZ twins were very different from DZ twins in their reaction to the SCO con-dition. Although they appeared to be competing, the extra rounds played could notreflect competition for more marbles. They seemed to be competing for the positionwhere the marble fell.

Hierarchical log linear analysis was applied to the SCO frequency data in Table 4.The best fitting model was found to be a saturated model involving three factors: equal-ity, zygosity, and cooperative/competitive behavior. Tests of partial association showed

that cooperative versus competitive behavior is related to equality (

x

2

5

8.90,

p

,

0.01). Only five twins actually cooperated for unequal reward. However, regardlessof cooperative/competitive behavior, more MZ twins are equal than unequal in mar-ble positioning (23 vs. 8), compared to DZ twins who do not show this pattern (14 vs.10) (

x

2

5

8.21,

p

,

0.01). In addition, MZ twins are less cooperative than DZ twins,whether or not the outcome is equality in marble positioning (

x

2

5

27.06,

p

,

0.01).Equality is itself a significant main effect (

x

2

5

6.701,

p

,

.01). Regardless ofzygosity of twin pairs and whether they play cooperatively or competitively, morepairs achieve equality as opposed to inequality in terms of marble positioning.

In summary, under the SCO condition, MZ twins competed whereas DZ twinscooperated. Somehow it was more difficult for MZ than DZ twins to arrive at equalityin positioning of marbles, even though equality of actual reward was unavoidable.

Turning to the RA condition, there were now opportunities to gain unequal re-ward. The MP ratio corresponds to the actual ratio of rewards. It is clear by inspec-tion that equality is preserved. Fifty-two of 55 pairs showed equal reward. However,DZ twins took significantly more trials than MZ twins to arrive at this equality, inmarked contrast to the SCO condition. Given reward equity, the distribution of co-operation and competition reverses as a function of zygosity, (

x

2

5

8.42,

p

,

0.01,for the frequencies in the upper right quadrant of Table 4).

Table 4. Summary of MZ and DZ Distributions for Competitive and Cooperative Behavior with Reference to Equal or Unequal Marble-Position (MP) Outcome Ratios

SCO RA

Condition MZ DZ MZ DZ

Equal MP outcome (6:4 or 5:5)Cooperative (< 20 rounds played) 4 14 24 7Competitive (> 20 rounds played) 19 0 6 15Totals 23 14 30 22

Unequal MP outcome (7:3, 8:2, 9:1, or 10:0)Cooperative (< 20 rounds played) 1 4 0 0Competitive (> 20 rounds played) 7 6 1 2Totals 8 10 1 2

Note. For SCO, ratios reflect only which ends of the apparatus were used to position the marble. For RA, the ratio alsoreflects the actual reward distribution.

Cooperation and Competition in Twins 407

Zygosity and Parents’ Beliefs

Analysis of the transformed scores for mean rounds played in SCO and RA re-vealed no significant difference related to whether the parents were correct or un-sure/wrong about their twins’ zygosity. Twins whose parents were correct abouttheir zygosity (n 5 24 MZ, 15 DZ) behaved no differently from twins whose parentswere mistaken or unsure (n 5 7 MZ, 9 DZ); t , 1 for all comparisons.

Order Effects

The SCO always preceded the RA condition, so results from the latter are notindependent of the former. The original fear was that SCO might prime DZ twins to-wards cooperative, turn-taking behavior and neutralize any effects the RA conditionmight have. In fact, DZ twins were significantly more competitive in RA even afterbehaving cooperatively in SCO. In addition, the unexpected finding that MZ twinswere competitive in the SCO condition could not have been the result of priming.

Did MZ twins quickly resolve the RA game simply because they had wasted alot of time fruitlessly during the preceding SCO game? Apparently not, because therounds played in SCO and RA by MZ twins were significantly positively correlated(r 5 0.70, p , 0.001). MZ twins who were involved in longer conflicts during theSCO condition were also the ones who were involved in longer conflicts in the RAcondition, and vice versa.

DISCUSSION

It is clear that MZ twins are not always more cooperative than DZ twins, and thatcompetition can occur despite unavoidable equality of tangible reward. The resultsare not explainable as order effects, and parental zygosity belief made no differenceto the pattern of results.

We suggest that the explanation of these patterns lies in the differential stabilitycharacteristic of dominance relations in the two types of twin. Von Bracken (1934)noticed that MZ twins lacked separate leader and follower roles and, by implication,tended not to have stable dominance–submission relationships. In the current study,some mothers actually mentioned this of their twins. These MZ twins were said to“interchange their personalities,” i.e., interchange elder twin (leader) status and taketurns in the role. The idea of interchanging elder twin status may reflect Asian con-vention whereby birth status is perceived as an important leadership trait. Somemothers of MZ twins indicated that their twins took turns at being elder twins,although they knew who was actually the elder according to birth order. The one forthe time being designated the younger would address the older one as “elder brother/sister” and defer to that twin. Confusion over elder twin status was particularlynoticeable in Malay families, where it is a cultural belief that the first twin to beborn is the younger twin, as the elder twin allows the younger to be born first. In

408 C. Y. Loh and J. M. Elliott

some of these families, the parents themselves would accord the second-born twinolder twin status. This often created some confusion for the young twins, becausesome people outside the family would refer to them in the opposite manner.

We suggest that MZ twins have no preestablished dominance–submission rela-tionship, not as a matter of choice, but because of the difficulty of establishing one.Dawkins (1989) highlighted the need for a stable asymmetry to exist before domi-nance–submission could function as a good strategy to minimize aggression. Itwould not be surprising if MZ twins find it difficult to maintain a one-directionaldominance for long periods of time. Rather than supposing that MZ twins strive forequality (Segal 1984, following Von Bracken 1934), we suspect that MZ twins haveequality as a necessary result, not a goal. Equality avoids conflicts and, normally,lacking a stable dominance hierarchy, MZ twins will show equality when there isthreat of high-risk conflict. Where conflict is not great and little risk can be incurred,however, dominance testing can still occur between MZ twins.

Among DZ twins, in contrast, stable inequality rather than unstable equalityprevails (Segal 1984; Von Bracken 1934). Once established, the dyad relationshipcould retain stability, but a novel situation with possible differential reward couldexcite conflicts because of the need to maintain or challenge dominance.

In the SCO condition, the equality of rewards was fixed. DZ twins should notexperience any conflicts in the situation, as they previously resolved issues of domi-nance and there can be no difference in rewards. There was essentially nothing tofight over. Therefore, they responded by simply completing the task in the most ex-peditious manner allowed by the design. In terms of marble positioning, most DZdyads engaged in reciprocity, whereas a small number engaged in dominance be-havior, but in either case they were consistently cooperative—the SCO task wascompleted swiftly. It was as if the twins had prior contracts about how to resolve agame like this.

MZ twins may have experienced the SCO situation somewhat differently. Nothird party could benefit from a fight between co-twins, and there is no risk of losingany of the promised rewards while fighting. In this relatively low-risk environment,the twins may have found that they could consider the unresolved issue of domi-nance in their relationship without tangible consequences, and accordingly theycompeted for marble position. Most MZ dyads resolved the marble positioning withequality, whereas a small number resolved the conflict with dominance. This sug-gests that the former remained unable to resolve the dominance issue and fell backon a strategy based on equality, whereas the latter were successful in resolving it.

Under the RA condition, in contrast, equality of tangible marble rewards is notguaranteed. To maintain equality, the only effective strategy is to engage in recipro-cal altruism, i.e., to take turns. This is probably a familiar situation for MZ twins.There is potentially intense and unfruitful conflict for tangible rewards. Therefore,they reverted to their normal mode of cooperative behavior and swiftly divided theresults equally between themselves. This behavior was predicted for MZ twins inthe initial hypothesis, which did not take dominance into account.

DZ twins, likewise, reacted in a way that is predictable if it is true that their ac-tions are driven by reward opportunities. The RA situation created a situation where


there was an opportunity for the underdog co-twin to withhold cooperation andrefuse to budge when asked to forfeit a potentially equal share in rewards. It is no-ticeable that most DZ twins ended up with equal rewards anyway, even after playingnumerous rounds indicative of competitive behavior.

In summary, we think that the marble-pull apparatus was eliciting two differentsets of behaviors based on two sets of perceived rewards. Tangible marble rewardsacted to elicit predictable behaviors based on relatedness. The intangible reward ofmarble positioning in the SCO situation interacted with issues of dominance to elicitunexpected competitive behavior in MZ twins. We predict that MZ twins will varyin their reaction to the games as a function of the stability of their dominance rela-tion, which could be tested by obtaining systematic data from parents on variation insocial roles in twin dyads over time.

Parents’ Reactions to Twins

MZ twins spend significantly more time together and have more shared activitiesthan DZ twins (Rowe 1981). In the present study, however, parents of MZ and DZtwins alike responded that their twin children spend all their time together. Apartfrom some pairs attending separate classes, twins were reported to be never sepa-rated. This may be because Singapore parents generally are very concerned withtheir children’s academic achievement and much of children’s out-of-school hoursare likely to be spent on homework, additional tuition, or other educational pro-grams.

Enquiries about shared leisure activities likewise revealed no noticeable differ-ence in the responses of parents of MZ and DZ twins. Parents generally preferred theirtwin children to participate in the same activities, such that if one twin expressed in-terest in an activity, both children were enrolled. Conversely, there were parentswho started off intending to cater to the individual interests of their twin children,only to find that the children chose to participate in the same activities anyway.

We thus found no support for the idea that parents systematically treated theirtwins differently as a function of zygosity. For example, parents of both MZ and DZtwins stated that they liked dressing the twins alike because there was less to fightover if all their belongings were the same. In fact, most parents did not bother toconfirm their twin children’s zygosity at birth, and many of them were not awarethat there were two types of twins. It is clear, moreover, that parental beliefs abouttheir twins’ zygosity did not predict the behavior of their twins in the experimentalgame. Twins whose parents were wrong about their zygosity behaved in the sameway as those whose parents were correct.

So far as the twins themselves were concerned, DZ twins generally were moreaware of less noticeable factors, such as similarities in their birth dates, as relevantcriteria for twinship. MZ twins were more aware of physical similarities and ap-peared to be more enthusiastic about being twins than DZ twins. However, mosttwins indicated that they liked playing with their co-twins. They sometimes fought,but still preferred to be together rather than separated.

410 C. Y. Loh and J. M. Elliott

CONCLUSION

The RA condition offers support for the existing view that MZ co-twins are moremutually cooperative towards each other than DZ co-twins when taking turns toacquire tangible resources. This difference could not be related to differences inreported parental treatment and beliefs in their children’s zygosity, nor did it appearto be related to differences in factors such as shared activities.

However, the unexpected but clearcut result in the SCO condition suggests thatMZ twins are more competitive than DZ twins when the situation guarantees equal-ity in material rewards and there are no serious consequences of disagreement. Situ-ational factors are important in the study of cooperative and competitive behaviorsin twins. We suggest, speculatively, that the existence of uncertain or inconsistentdominance might account for this result.

Financial support from the National University of Singapore (Grant RP 930026 to J.M.E.) is gratefullyacknowledged. We are also grateful for the support of the Ministry of Education, Singapore (Ref ESS/93/10), and all participating schools and families. Thanks to the Editors and an anonymous reviewer for theirhelpful comments on an earlier draft of the manuscript.

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