News, Views and Comments

NEWS, VIEWS AND COMMENTS

Nancy L Segal

Department of Psychology, California State University, USA

Forgotten Twins ful, so several insightful studies (both might amplify the twins’ existing past and present) may escape noti ce. genetic differences. Recent, rapi d i ncreases i n di zygoti c DZ tw i n i nvesti gati ons have expl ored Neverthel ess, the tw i ns’ geneti c het- (DZ) twinning rates, largely a product associ ati ons betw een fraternal tw i ns’ erogeneity would be the most common of the new assi sted reproducti ve tech- blood group similarities and perceived explanation for their diverging devel- 8 9 nologies (ART), have refocused atten- zygosi ty, i ntel l ectual resembl ance opment. Few i nvesti gators rai se super- 10 ti on on the pl easures and pi tfal l s of and physi cal appearance. More fecundation as an explanation because multiple births. Formerly childless recent work on DZ twins includes superfecundation is assumed to be couples are gratified by their ‘instant genomi c anal ysi s for i denti fyi ng rare. Contrary to conventional fami l i es’, w hi l e physi ci ans grappl e regi ons w i th genes rel evant to age- thought, I believe this remains an rel ated chroni c di seases, such as di a- untested assumpti on, and urge with developmental delays common 11 to premature twins and triplets. Even betes and osteoporosis. renewed respect for the possibility while DZ twinning rates exceed mono- The non-scientific world correctly that superfecundation occurs more zygotic (MZ) twinning rates (a pattern perceives that most fraternal twins frequently. Evidence from other that generally characterized natural lack the striking behavioral and phys- sources supports this view. Estimated ical similarities of identical twins. non-paternity rates range from twinning prior to ART), the majority 13 does not rule! Fraternal twins remain Fraternal tw i ns are, consequentl y, l ess 5–30%, fraternal twinning may be captivating to general audiences than higher among illegitimate births than the forgotten pairs in many scientific 14 and non-scientific undertakings. For their identical counterparts. Almost legitimate births and coital fre- exampl e, the three earl y reared-apart exclusive attention to identical twins quency may be hi gher among w omen in educational programs and other beari ng fraternal tw i ns than among twin studies did not include DZA twin 15 pai rs i n thei r sampl es.1–3 The Minne- public events undermines fraternal women bearing identical twins. It twins’ crucial role in the scientific has been suggested that one i n 12 fra- sota Study of Twins Reared Apart, enterprise. We should never forget that ternal twin pairs are superfecundated, directed by Dr Thomas J Bouchard Jr, i denti cal tw i n resembl ance i s l ess and that bipaternity occurs in one in w as the fi rst i nvesti gati on of tw i ns meani ngful than i denti cal tw i n resem- 400 pai rs but these esti mates are reared apart to systemati cal l y i ncl ude 16 blance compared with that of fraternal preliminary. separated fraternal s.4 Beyond the labo- twins. Furthermore, within the fra- Superfecundated twins attract undi- ratory, identical twins have remained ternal population is a wealth of exotic vided attention when different fathers the preferred parties in documentaries and undiscovered couples that may are i nvol ved. The fi rst sci enti fi c taste and works of fiction. help solve persisting behavioral rid- of these extraordi nary pai rs w as pro- These si tuati ons are understand- dl es. Reapprai sal of sel ected tw i n vided by a famous 1810 report of twins able, but unfortunate. They are under- research methods and fi ndi ngs may be w hose contrasti ng appearances standable because identical twin dif- warranted once the fraternal twin reflected their fathers’ differing eth- ferences provi de persuasi ve evi dence story fully unfolds. ni ci ti es. These remarkabl e pai rs al so of environmental effects on human Fraternal tw i ns’ compl exi ti es begi n occur in Greek mythology, although devel opmental trai ts. Researchers are, with superfecundation, the fertiliza- vari ed versi ons of these events exi st.17 how ever, documenti ng i ncreasi ng ti on of tw o eggs by tw o spermatozoa at Zeus, taki ng advantage of A mphi tyr- numbers and vari eti es of geneti c di f- different times within the same men- on’s absence, assumed hi s l i keness to ferences betw een some i denti cal strual cycl e (usual l y three or four days court his wife, Alcmeme. When her co-twins. These di fferences range from apart).12 Such co-twins are infre- real husband returned several hours variations in X chromosome inactiva- quently detected because their l ater he w as surpri sed by hi s w i fe’s tion patterns in some identical chronol ogi cal age di fference i s sl i ght, lack of enthusiasm and she was baffled 5 (femal e) pai rs to discordance for med- and because their behavioral and by hi s ardor. Tw i ns resul ted from these 6 i cal di sorders and congeni tal anoma- physi cal di fferences are easi l y asso- unions, Hercules (son of Zeus) and 7 lies in others. Rethinking the mean- ciated with their genetic differences. It Iphicles (son of Amphitryon). Zeus ing of identical twin differences may i s, how ever, al so possi bl e for eggs to be al so transformed hi msel f i nto a sw an be w orthw hi l e i n some cases. Fra- rel eased at somew hat greater i nterval s to seduce Leda, wife of Tyndareus. ternal twin differences reflect both (eg 1–2 w eeks), agai n al l ow i ng the Two sets of twins were conceived on geneti c and envi ronmental factors, so possibility of separate conceptions. the same night, Pollux and Helen fraternal twin studies generally offer These tw i ns are more l i kel y to attract (children of Zeus) and Castor and l ess cl ear concl usi ons as to i nfl uences attention because their early devel- Clytemnestra (children of Tyndareus), affecti ng measured phenotypes. Case opmental differences may be more yielding several superfecundated sets. reports and formal investigations lim- pronounced. For example, the rela- Medical science has never docu- i ted to fraternal tw i ns are l ess pl enti - ti vel y greater prematuri ty of one tw i n mented the birth of two pairs of twins

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(quadruplets) in which different two fathers were involved was not patterns.4 Hi gh i denti cal tw i n resem- fathers were responsible for different addressed, but the i nvesti gators bl ance and l ow fraternal tw i n resem- pairs, but this is theoretically possible. acknowledged the difficulties in find- bl ance for measured trai ts may refl ect Medical journals do describe intrigu- ing superfecundated twin pairs in emergeni c effects. Emergenesi s w oul d ing superfecundated twin pairs that Japan’s physically homogenous pop- not affect i denti cal tw i n resembl ance warrant further study. ul ati on. They urged fi rst tri mester because identical twins share 100% of Twins with different fathers were measurement of CRL to di scover the thei r genes. How ever, emergenesi s identified in 1982 when a Caucasian true frequency of this event. w oul d reduce fraternal tw i n resem- mother sought medical attention for Two bipaternal superfecundated blance given the low (but non-zero) her four-year-old fraternal twin girls twin pairs were identified in China in probability of both co-twins inheriting w ho vari ed i n appearance, compl ex- 1994, following paternity testing – a the same unusual gene combination. ion and features of fetal hydantoin w i fe’s extramari tal affai r and an i l l egi t- Superfecundated twins with separate syndrome.18 Fetal hydantoin syn- imate twin birth, rather than apparent fathers differ, by descent, for pater- drome, characteri zed by skel etal and tw i n di fferences, suggested di fferent nally derived genes so their matching crani ofaci al abnormal i ti es, has been fathers, thus prompting genetic test- on emergeni c trai ts i s vi rtual l y i mpos- tied to prenatal exposure to pheny- ing.21 I al so recal l a new s report consi bl e. A gai n, i ncl udi ng such tw i ns i n toin, a substance the twins’ mother cerning black twins with different smal l tw i n sampl es or si ngl e case received for epilepsy treatment at fathers, a finding revealed only reports coul d yi el d mi sl eadi ng esti - age13. Twins with different fathers because geneti c anal ysi s w as needed mates of geneti c i nfl uence on compl ex were also identified in 1997 when the to diagnose one twin’s medical condi- phenotypes. Now that DNA analysis is l egal father requested paterni ty i denti - tion. Other superfecundated twins becoming more common and less fication.19 The female twins, delivered may well go unnoticed in the absence expensive it may be worth testing at 35 w eeks gestati on, show ed a of marital or medical considerations twins and their parents in future stud- 150gram weight difference. The that justi fy geneti c testi ng. i es. Researchers may w ant to thi nk smal l er tw i n (bi rthw ei ght = 750 g) What does superfecundation imply seriously about whether and how to experi enced anemi a and septi c ri sk for twin studies? A concern is that include such sets in ongoing studies. due to her prematurity. When pater- di scordance i n seemi ngl y ordi nary fra- How can superfecundated twins nity was excluded for one twin and it ternal tw i ns mi ght be associ ated w i th enhance understanding of human was determined that neither infant separate conceptions. Co-twins con- behavior? Evolutionary psychology had been rel eased to the w rong fami l y, cei ved apart coul d show greater di ffer- suggests several i mportant w ays i n the mother admitted to an extramarital ences for some traits than co-twins which fraternal twin–superfecundated rel ati onshi p. Thi s case w as l i sted as concei ved together, even w hen fathers tw i n desi gns may yi el d new i nsi ghts the seventh occurrence of superfe- are shared. Fi rst, gi ven a ten-day i nto soci al rel ati ons. There i s abun- cundation involving separate fathers, between-conception interval, one twin dant evidence that identical twins but the number is actually higher. The w oul d enter the embryoni c phase (at show cl oser soci al rel ati ons than fra- 1982 study cited above documented the start of the second gestati onal ternal tw i ns.12 Many proximal 19 cases, 11 i nvol vi ng raci al bi patern- week) while the co-twin would be (i mmedi ate) expl anati ons are avai l a- i ty, and more recent case reports are w i thi n the zygoti c phase. Embryoni c bl e, such as i denti cal tw i ns’ greater available. I have identified additional hearts, eyes and l i mbs are especi al l y physical and behavioral similarities cases i n the course of provi di ng l egal sensitive to teratogens during the third and/or greater opportunities for assi stance to families with twins, and and fourth w eeks of gestati on.22 In shared experi ences. These expl ana- am unaw are that these pai rs have been contrast, zygotes are rel ati vel y unaf- tions provide useful, but partial views appended to the scientific literature. fected by these agents unl ess effects of behavior that may be enriched with The presumption that superfecunda- are so severe as to be l ethal . Secondl y, reference to evolutionary considera- tion is a rare twinning phenomenon ordi nary fraternal tw i ns share, on ti ons. For exampl e, fi ndi ngs on tw i ns’ seems unjusti fi abl e i n the absence of average, one-hal f of thei r genes, w hi l e soci al rel ati onshi ps are consi stent systematic study. twins conceived by different fathers with William Hamilton’s (1964) theory Consider, too, the difficulty of share one-fourth of thei r genes (as do of i ncl usi ve fi tness.23 Hamilton rea- detecting superfecundated twins in half-siblings). Ordinary fraternal twins soned that natural selection favors populations with limited physical can share domi nance vari ance, w hi l e alleles predisposing individuals to trait variation – superfecundation half-siblings cannot because they have behave in ways favoring the transmis- mi ght easi l y escape noti ce because only one common parent. The pres- sion of those alleles into future genera- fathers would match across character- ence of superfecundated twins in tions. Alleles influencing individuals istics.20 Superfecundation was sug- modest si ze tw i n sampl es coul d to favor others likely to carry replicas gested by ul trasoni c exami nati on of i ncrease i denti cal –fraternal tw i n di f- of these al l el es i s an i ndi rect means by Japanese tw i ns at 10 w eeks’ gestati on, ferences for some traits, thus inflating w hi ch these al l el es can be represented reveal i ng a tw o-w eek di screpancy i n heritability. subsequentl y: ‘A gene causi ng i ts pos- co-twins’ crown-to-rump length Emergenesi s i s a term coi ned by Dr sessor to gi ve parental care will leave (CRL). The i nteresti ng feature of thi s David Lykken at the University of more repl i ca genes i n the next genera- case w as that the tw i ns’ prenatal si ze Minnesota to describe traits influ- tion than an allele having the opposite difference, rather than their postnatal enced by complex configurations of tendency. The sel ecti ve advantage appearance, raised the possibility of genes across many loci, eg unusual may be seen to l i e through benefi ts superfecundation. The chance that musical talent and selected brain wave conferred indifferently on a set of

Twin Research

rel ati ves each of w hi ch has a hal f pected that her current and former mal or superior folk physics (ability to chance of carrying the gene in ques- husbands w ere each responsi bl e for understand physical objects with ref- tion’ (p. 1).23 Therefore, i t pays (geneti - one twin.27 Even from birth, the erence to thei r causal or mechani cal cal l y speaki ng) to di rect benefi ts appearance of each i nfant gi rl sug- properti es).29 The first twin study of toward close relatives. Recognizing gested di fferent fathers, a suspi ci on early understanding of mind investi- w ho one’s rel ati ves are i s essenti al and confi rmed by geneti c testi ng. The for- gated geneti c and envi ronmental i nfl u- various mechanisms for doing so have mer husband initially sought custody ences on young children’s ability to been proposed. Compari ng soci al of ‘his twin’. He was eventually comprehend the behavior and inten- rel atedness betw een fraternal tw i ns granted limited visitation rights and tion of others.30 Participants included who share fathers and those who do both twins remained in their mother’s 61 monozygotic (MZ) and 58 same-sex not would allow further tests and care. Similar events unfolded in a dizygotic (DZ) three-year-old twin applications of inclusive fitness more recent case i n w hi ch I provi ded pairs enrolled in the TRACKS Twin theory. expert testi mony. The most curi ous Study in Great Britain. Twins com- Research show s that chi l d abuse i s feature of these cases w as the effort pl eted ei ght fal se-bel i ef tasks and tw o el evated i n fami l i es w i th step-chi l - made by fathers on behalf of their own decepti on tasks, as w el l as the vocabu- dren24 and in families with twins.12 twin children – had the birth involved l ary and comprehensi on subtests of Evol uti onary psychol ogi sts M arti n their own single-born child they may the Stanford–Binet Intel l i gence Scal e. Dal y and M argo Wi l son have asserted have been l ess commi tted to the ensu- A sampl e i tem for testi ng fal se bel i ef that ‘Investing resources in non-bio- i ng l egal contest. Perhaps shame and requi red chi l dren to i nspect a seri es of logical children may lead to “violent lowered self-esteem from their part- drawings: two children stand beside a resentment” on the part of step-par- ner’s i nfi del i ty l ed to ‘asserti ons of basket and a box; one chi l d pl aces a ents’ (p. 304).25 Tw i n researchers have paternity’ in the form of securing ball in the basket; that child exits; the l i nked i ncreased fami l y stress and rights to their own child. Further remaining child removes the ball from reduced chi l d heal th to i ncreased eval uati on of such cases and control s the basket and places it in the box; the abuse of twin infants and twin chil- is needed to evaluate this notion. first child returns. Twins are then dren, relative to non-twins. (Even An important caveat to much of the asked: ‘Where will Sally look for the w hi l e tw i ns are overrepresented i n above is that the strikingly different ball?’ sampl es of abused chi l dren, most par- appearances of some fraternal tw i ns Geneti c i nfl uence w as i ndi cated for ents do not abuse their multiples.) are not necessari l y si gns of super- both the theory of mind tasks (MZ Ordinary fraternal twins can help test fecundation. The contrasting physical ri = 0.66, DZ ri = 0.32) and verbal the hypothesis that paternity uncer- features of some twins born to inter- measures (tasks (M Z r i = 0.74, DZ tainty may underlie differential bond- racial couples remind us that genetics ri = 0.56). Most of the genetic influ- i ng and/ or abuse of one fraternal tw i n. underlies both similarities and difference on understanding of mind (66%) Specifically, fathers may perceive ences among fami l y members. I have proved independent of verbal ability. reduced physical and/or behavioral encountered several such pairs over Non-shared envi ronmental i nfl uences resembl ance to one tw i n, rel ati ve to the years, one set the product of a were also substantially implicated in the child’s co-twin, thus affecting their Black mother and Caucasian father, the development of this behavior. The evolving relationship. Such parental and the other the product of a Black i nvesti gators concl uded that genes favori ti sm need not operate con- mother and White-Filipino father. may contribute to the development of sciously – people may be predisposed There are many other captivating social understanding, thus affecting to be sensitive to who their kin are vari eti es of fraternal tw i ns that many features of one’s soci al i nter- since caring for kin improved survival deserve careful study. They include actions. It would be worth considering throughout human history. pol ar body tw i ns, superfetated tw i ns, whether MZ twins show superior per- Superfecundated twins with differ- chimeric twins and male–female formance on understanding of mind ent fathers offer a more stringent test twins. Future columns of this journal tasks, relative to DZ twins and non- of differential bonding between will explore the biological and beha- twins. This group difference would be fathers and one twin. (Note that child vi oral aspects of these sets and w hat reasonabl e, gi ven some i denti cal thei r presence i mpl i es for tw i n data tw i ns’ nonverbal (gestural ) commu- abuse in families with twins more 26 and desi gns. nication patterns and exquisite sensi- often involves one twin than both. ) tivity to their co-twins’ intentions and A n advantage of usi ng tw i ns to under- feel i ngs. M Z tw i ns experi enci ng di ffi - stand differential child abuse is that cul ti es i n these tasks mi ght be l ess fami l y factors associ ated w i th abuse Twins’ Minds and Bodies close socially to their co-twins than (eg financial concerns, marital difficul- Theory of mind hi gh-scori ng tw i ns. ti es) are hel d constant because tw i ns enter the family at the same time. Twin studies show that autism, a Births of some superfecundated di sorder i nvol vi ng severe soci al Cerebral mirror-imaging twins with different fathers have impairment and affecting 5–10 out of evolved into curious custody battles. 10000 children, is influenced by Ectodermally derived traits (eg hair These cases are ri fe w i th raw materi al geneti c factors.28 Autistic children whorl and dermatoglyphic patterns) for expl ori ng evol uti onary-based appear to have abnormal development show mi rror-i magi ng effects i n hypotheses surrounding paternity of their folk psychology (ability to approximately 25% of identical twin issues. An illustrative 1996 case con- understand other people’s behavior by pairs. Studies investigating cerebral cerned an A ri zona w oman w ho sus- their intentions), while showing nor- mirror-imaging of cognitive function

Twin Research

in MZ pairs have produced a mixed shared envi ronmental i nfl uence. Con- multiple birth rates.38 (Multiple births picture. Lack of consistent findings tinued study of genetic, environ- from one pregnancy were considered may reflect failure to organize hand- mental and gene-environment inter- one l i ve bi rth.) A ssessi ng outcomes discordant pairs by source of left- acti ons rel evant to eati ng di sorders i s from these procedures i s i mportant for handedness (i e bi rth trauma or recommended. (Note: earl y reared- minimizing the well-known risks delayed zygotic division)31 and/or apart twin studies did not include associ ated w i th mul ti pl e pregnanci es. i neffecti ve methods for assessi ng mi r- di scussi on of eati ng probl ems; rel e- Some nati ons l egal l y restri ct the num- ror-imaging of cerebral function. A vant data from ongoi ng anal yses ber of transferred embryos to three, but recent case report illuminates this would be informative.) the Uni ted States has been rel uctant to issue and much more. A pair of hand- (2) Language impairments. What adopt a formal policy out of respect for discordant MZ female twins com- tw i ns tel l us. Thi s summary asserts the inviolability of patient–physician pl eted a l anguage task and a mental that tw i ns’ l anguage defi ci ts are w el l relationships. rotation task, activities that differ- known, yet their nature and complex- Data were gathered from 300United enti al l y engage the l eft and ri ght hemi - i ty have been l ess appreci ated.35 The States centers, esti mated to represent 32 spheres. Brain activity was mon- twin situation, ie growing up in close over 95% of all 1996 cycles reported i tored by magneti c resonance i magi ng proxi mi ty to a same-age si bl i ng, i s for that year. (A cycl e begi ns w hen a (MRI) while tasks were performed. regarded as the pri mary factor restri ct- woman starts fertility medication or The dextral co-twin showed pro- ing twins’ communicative abilities. ovarian monitoring with the intention nounced left dominance during the Speci fi cal l y, tw i ns’ reari ng ci rcum- of transferring embryos.) Findings l anguage task w hi l e her si ni stral stances resul t i n reduced verbal i nter- w ere strati fi ed by maternal age and co-twin showed the reverse pattern. action with parents and atypical lan- w hether addi ti onal embryos (mai n- The dextral tw i n show ed greater ri ght guage forms. The author asserts that tained through cryopreservation) were than left hemisphere activity during greater attenti on to i ndi vi dual cases avai l abl e. The ri sk of mul ti pl e bi rth the mental rotations task, while her would extend understanding of factors following IVF varied with the mother’s sinistral co-twin agai n show ed the relevant to twins’ speech develop- age and number of transferred reverse pattern. These tw i ns had expe- ment, such as prenatal and perinatal embryos. Live birth rates were 43% rienced a nontraumatic birth so it is bi rth hazards, geneti c predi sposi ti on and 36%, respectively, for women l i kel y that thei r handedness di scor- for l anguage di sorders and speci fi c 20–29 and 30–34 years of age w hen dance refl ected embryol ogi cal events. features of tw i ns’ reari ng si tuati ons. two embryos were transferred. Live The authors of this study raised the Given the importance of good lan- birth rates were consistently lower for possibility that cerebral organization guage skills for successful soci al and women older than 35. Multiple birth differences in MZ co-twins may intellectual functioning, this language rates w ere al so hi gher for younger expl ai n di fferences i n di sorders such review is a welcome addition to the w omen and hi gher w i th i ncreased as schi zophreni a, auti sm and dysl exi a. twin literature. The author recognizes number of embryos. For exampl e, A 1977 study by Boklage show ed that the different twin types undergo among women 20–29 years of age mul - reduced concordance for schizophre- different speech-relevant experiences, tiple birth rates were 22.7% and nia in right-left and left-left MZ twins, 45.7% with transfer of two and three 33 although this point would have been compared with right-right MZ twins. more effective had it occurred earlier embryos, respecti vel y. In contrast, Col l eagues’ fai l ure to repl i cate hi s in the review. It will interest readers to among women 40–44 years of age mul - findings directed attention away from l earn that some i nvesti gators have tiple birth rates were 10.8% and 25% this perspective on MZ co-twin differ- i denti fi ed l anguage skills in which w i th transfer of tw o and fi ve embryos, ences i n psychi atri c syndromes. It twins outperform non-twins, eg twins respecti vel y. Embryo qual i ty w as not may be time to revisit this issue in may understand one another better associ ated w i th mul ti pl e bi rth ri sk, light of vastly improved experimental than do unrelated peers. Two recent- but w as associ ated w i th i ncreased l i ve technologies. l anguage studies not included in this birth rates when fewer embryos were summary show ed i ncreased turn-tak- transferred. ing by twins in twin–mother triads Multiple review 36 than in single twin–mother dyads, (1) Eati ng di sorders revi si ted. Tw i n and superior quality and quantity of studies of anorexia nervosa and buli- speech by twins than by singletons.37 Insights and Instincts These contradi ctory, but encouragi ng mia have yielded a mixed picture with Parallel lives respect to the relative contributions of fi ndi ngs shoul d sti mul ate research geneti c and envi ronmental i nfl uences. efforts to clarify the complexity sur- Deni se LaChance i s the general man- A recent comprehensive review rounding twins’ language l earni ng ager of Li ve on Li ne, a company broad- reported w i del y rangi ng esti mates of environments. casting events live on the Internet.39 genetic influence for both conditions (3) Multiple birth risk and IVF. Her i denti cal tw i n si ster, Debra, i s the (anorexia: 0–70%; bulimia: 0–83%).34 Researchers from the Di vi si on of chief operating officer of that com- Inconsistencies across studies were Reproductive Health at the Centers for pany. The 46-year-old identical twins variously explained by differences in Di sease Control and Preventi on, i n recently moved into adjoining New sampl e si ze and representati veness, A tl anta, reported an extensi ve survey York City town house apartments with diagnostic criteria for bulimia, adher- of associations between a number of thei r respecti ve spouses. ence to the equal environments embryos transferred duri ng i n vi tro Their lives reveal striking parallels, assumpti on and ability to detect fertilization (IVF), and live birth and as w el l as occasi onal detours. Both

Twin Research

twins graduated from Boston Uni- Searching for lost twinship 3 Juel-Nielsen N. Individual and Environ- versity, after which Denise remained ment: Monozygotic Twins Reared there to attend law school, while her M any tw i ns reared apart i nvest con- Apart. International Universities Press: twin enrolled in New York Univer- siderable time and expense in locating New York, 1966. the twin sibling from whom they were 4 Lykken DT. Happiness: What Studies si ty’s busi ness admi ni strati on pro- on Twins Show us about Nature, Nur- gram. Denise relocated to New York separated in infancy. Unfortunately, not al l separated tw i ns are successful ture, and the Happi ness Set Poi nt. upon compl eti ng her l aw degree and Golden Books: New York, 1999. w hi l e the si sters grew cl oser soci al l y, in this quest, a situation difficult for 5 Derom C. X chromosome inactivation their careers took different tracks – many to accept. Fraternal twin André patterns in monozygotic twin girls Deni se w orked i n heal th care (as an Carmichael from Brooklyn, New York, (abstract). Twin Res 1998; 1: 101. attorney) and Debra worked in fash- is now at the center of a tragic case 6 Korenke GC, Fuchs S, Krasemann E, i on. When Deni se sought a job change involving lost twinship. Childhood Doerr HG, Wilichowski E, Hunneman rumors of a tw i n si ster, vague recol l ec- DH, Hanefeld F. Cerebral adrenoleuko- she joi ned her tw i n i n the fashi on dystrophy (LAD) in one of monozygotic world, after which Debra moved on to tions of ‘a baby’ and commitment to twins with an identical ALD genotype. establish Live on Line in which both finding a missing twin were ultimately Ann Neurol 1996; 40: 254–257. tw i ns are now acti ve. Thei r l i fe hi story responsible for his learning the diffi- 7 M astroi acovo P et al . Congeni tal mal - affi rms once agai n that tw i n studies of cult truth.41 In October 1999 it was formations in twins: An international occupational choice should focus on revealed that his mother (who had study. Am J Med Genet 1999; 83: tw i ns’ l ast or ‘best’ posi ti ons for accu- pl aced hi m i n foster care at age 9) had 117–124. rate assessment of job sel ecti on and fatal l y beaten hi s tw i n si ster, Lata- 8 Dumont-Driscoll M, Rose RJ. Testing the twin model: Is perceived similarity satisfaction. ni sha, w hen she w as onl y 3 years ol d. due to genetic identity? (Abstract). Carmichael is planning to file a law- Behav Genet 1983; 13: 531–532. sui t agai nst the ci ty for failing to 9 Bock RD, Vandenberg SG, Bramble W, Guilty until proven innocent protect hi s tw i n. Thi s case generated Pearson W. A behavi oral correl ate of Several years ago I w as contacted by enormous interest, making it likely blood-group discordance in dizygotic twins. Behav Genet 1970; 1: 89–98. an attorney representing an identical that additional facts will be revealed in coming months. Attention will, 10 Paksti s A , Scarr-Sal apatek S, El ston RC, twin whom the attorney believed was Si ervogel R. Geneti c contri buti ons to w rongl y accused of a cri me. Hi s tw i n hopefully, be directed to twin-related morphological and behavioral similar- brother had been convicted of similar aspects of thi s case as w el l as the i ti es among si bs and di zygoti c tw i ns: w rongdoi ngs several years earl i er, so heinous crime. Li nkages and al l el i c di fferences. Soc the concern was that jury members in Biol 1972; 19: 185–192. their small town would recall that 11 PR New sw i re. Gemi ni research com- event and be l ess than objecti ve i n plete GeneFind (TM) Study. 1 June 1999. rendering judgment. I am unaware of Wonderful Websites 12 Segal NL. Entwi ned Li ves: Twi ns and what eventually transpired in that What They Tell us about Human Behav- case, but a si mi l ar scenari o occurred • A comprehensive medical diction- ior. Dutton: New York, 1999. 40 in Los Angeles in December 1999. ary i s avai l abl e on-l i ne at: 13 Bellis MA, Baker MA. Do females pro- Truck driver Ray Nugent was impris- www.graylab.ac.uk/omd/in- mote sperm competition? Data for oned for 13 days on charges of bank dex.html humans. Animal Behav 1990; 40: robbery and attempted murder – but • A group that performs zygosity 996–997. police had mistaken him for his 14 MacGillivray I, Samphier M, Little J. diagnosis with many useful twin- Factors affecting twinning. In: MacGil- brother, Jay. The mistake occurred related links can be found at: livray I, Campbell D, Thompson B (eds). w hen an offi cer i denti fi ed Ray as Jay w w w.proacti vegeneti cs.com/ Twinning and Twins. John Wiley: Lon- after i nspecti ng hi s dri ver’s l i cense • A l i sti ng of reared apart tw i ns don, 1988; 67–97. photograph. The twins are believed to searchi ng for thei r co-twins, and 15 James WH. Coital frequency and twin- be fraternal, and while fraternal twins ning. J Biosoc Sci 1992; 24: 135–136. non-tw i ns searchi ng for tw i n rel a- general l y l ook l ess al i ke than i denti cal Note that an earlier study by a different tives and friends is located at: investigator did not report this trend. twins, some look very much alike. www.twinsworld.com/reuni- Nugent will receive $150 000 com- 16 James WH. The i nci dence of super- te.html fecundation and of double paternity in pensation from Los Angeles County. the general popul ati on. A cta Genet Med His difficulty was not with his twin Gemellol 1993; 42: 257–262. brother, but with law enforcement 17 Marcus PM. Superfecundation in officials. ‘They said that if I had a twin mythol ogy, hi story and poetry. 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