Thi s study r epor ts on zygosi ty deter mi nati on i n tw i ns of chi l dhood age. Par ents r esponded to questionnaire items dealing with twin similarity in physical characteristics and frequency of mi stak i ng one tw i n for another by par ents, r el ati ves and str anger s. The accur acy of zygosi ty di agnosi s w as eval uated acr oss tw i ns aged 6, 8, and 10 and acr oss par ents. I n addi ti on, i t w as exami ned w hether the use of mul ti pl e r ater s and the use of l ongi tudi nal data l ead to an i mpr ovement of zygosi ty assi gnment. Compl ete data on zygosi ty questi ons and on geneti c mar k er s or bl ood pr ofi l es w er e avai l abl e for 618 tw i n pai r s at the age of 6 year s. The method used w as pr edi cti ve di scr i mi nant anal yses. A gr eement betw een zygosi ty assi gned by the r epl i es to the questi ons and zygosi ty deter mi ned by DNA mar k er s/bl ood typi ng w as ar ound 93%. The accur acy of assi gnment r emai ned constant acr oss age and par ents. A nal yses of data pr ovi ded by both par ents and col l ected over mul ti pl e ages di d not r esul t i n better pr edi cti on of zygosi ty. Detai l s on the di scr i mi nant functi on ar e pr ovi ded. Twin Research (2000) 3, 134-141. Keyw or ds: tw i n zygosi ty, chi l dhood, questi onnai re, revi ew, di scri mi nant anal ysi s I ntr oducti onIn 1927, Si emens 1 suggested that the di agnosi s of zygosi ty i n tw i ns can take pl ace by eval uati ng the degree of resembl ance on geneti cal l y determi ned trai ts. Devel opment of thi s method resul ted i n the frequent use of questi onnai res, often i ncl udi ng those cri teri a ori gi nal l y proposed by Si emens, for exampl e.2 Several studi es have show n that the establ i shment of zygosi ty based on mai l ed questi onnai res i s of consi derabl e accuracy, w i th around 95% correctl y cl assi fi ed compared w i th bl ood or DNA typi ng. Studi es on the di agnosi s of zygosi ty by mai l ed questi onnai res are summari sed i n A ppendi x 1.

For a quanti tati ve geneti c study of pr e-school pr obl em behavi our s, w e have col l ected data w i th the Child Behavior Checklist for 2 and 3-year-old children (CBCL 2/3). Questi onnai r es w er e compl eted by mother s of 3620 tw i n pai r s: 633 monozygoti c mal es, 581 di zygoti c mal es, 695 monozygoti c femal es, 519 di zygoti c femal es and 1192 di zygoti c opposi te sex tw i n pai r s. The geneti c and environmental influences on the Externalising and Internalising Problem scales were estimated, si mul taneousl y w i th sex di ffer ences and si bl i ng i nter acti on effects. Geneti c factor s expl ai ned most of the observed variance for both Externalising and Internalising Problems. Cooperative sibling i nter acti ons w er e found for Exter nal i si ng Pr obl ems, i ndi cati ng that tw i ns r ei nfor ce each other 's behavi our. Sex di ffer ences i n geneti c ar chi tectur e w er e found for Exter nal i si ng Pr obl ems. Geneti c factor s expl ai ned 75% of the var i ance i n girls and 50% i n boys. Shar ed envi r onmental i nfl uences w er e onl y of i mpor tance i n boys. For both pr obl em scal es, non-shar ed envi r onmental factor s accounted for 25 to 32% of the var i ance. The obser ved var i ances of I nter nal i si ng Pr obl ems coul d be adequatel y expl ai ned by geneti c and nonshar ed envi r onmental factor s, w i th geneti c factor s accounti ng for 68% of the var i ance.Keyw or ds: pre-school chi l dren, probl em behavi our, Chi l d Behavi or Checkl i st, tw i ns, behavi our geneti cs I ntr oducti on A number of studi es i ndi cate that roughl y 10 to 15% of pre-school chi l dren show probl em behavi ours.

For a quanti tati ve geneti c study of pr e-school pr obl em behavi our s, w e have col l ected data w i th the Child Behavior Checklist for 2 and 3-year-old children (CBCL 2/3). Questi onnai r es w er e compl eted by mother s of 3620 tw i n pai r s: 633 monozygoti c mal es, 581 di zygoti c mal es, 695 monozygoti c femal es, 519 di zygoti c femal es and 1192 di zygoti c opposi te sex tw i n pai r s. The geneti c and environmental influences on the Externalising and Internalising Problem scales were estimated, si mul taneousl y w i th sex di ffer ences and si bl i ng i nter acti on effects. Geneti c factor s expl ai ned most of the observed variance for both Externalising and Internalising Problems. Cooperative sibling i nter acti ons w er e found for Exter nal i si ng Pr obl ems, i ndi cati ng that tw i ns r ei nfor ce each other 's behavi our. Sex di ffer ences i n geneti c ar chi tectur e w er e found for Exter nal i si ng Pr obl ems. Geneti c factor s expl ai ned 75% of the var i ance i n girls and 50% i n boys. Shar ed envi r onmental i nfl uences w er e onl y of i mpor tance i n boys. For both pr obl em scal es, non-shar ed envi r onmental factor s accounted for 25 to 32% of the var i ance. The obser ved var i ances of I nter nal i si ng Pr obl ems coul d be adequatel y expl ai ned by geneti c and nonshar ed envi r onmental factor s, w i th geneti c factor s accounti ng for 68% of the var i ance.Keyw or ds: pre-school chi l dren, probl em behavi our, Chi l d Behavi or Checkl i st, tw i ns, behavi our geneti cs I ntr oducti on A number of studi es i ndi cate that roughl y 10 to 15% of pre-school chi l dren show probl em behavi ours.