After scanning data from nearly 2000 mothers of fraternal twins, scientists from eight countries found two genes that increase a woman's ...SkiptomaincontentReadourCOVID-19researchandnews.ShareResearchershavefinallyfoundevidenceforgenesthatmakefraternaltwinsmorelikely.RealCreation/iStockphotoResearchershavelongknownthatwomenwhosefamiliesincludefraternaltwinsaremorelikelytogivebirthtotwinsthemselves,andthey’refinallystartingtofigureoutwhy.Afterscanningdatafromnearly2000mothersoffraternaltwins,scientistsfromeightcountriesfoundtwogenesthatincreaseawoman’schanceofhavingtwins—onethataffectshormonelevelsandanotherthatmayalterhowovariesrespondtothem.Thesecondofthesemayalsohaveimplicationsforwhysomewomenrespondbetterthanotherstoinvitrofertilization.Unlikeidenticaltwins—whoaregeneticallythesame—fraternaltwinsarenomorecloselyrelatedintermsofDNAthanregularsiblings.Butscientistsoftenliketocompareidenticalandfraternaltwinstounderstandhowmuchvariationinatraitisduetoenvironmentversusgenetics.Becauseofthat,severallargedatabasestracktwinsastheyage.In1987,ayoungbehavioralgeneticistatVrijeUniversiteitinAmsterdamnamedDorretBoomsmastartedtheNetherlandsTwinRegister,whichnowcontainsmorethan 75,000twins,triplets,andotherchildrenofmultiplebirths.Theparticipants’parentsallhadthesamequestion:Whydidtheyhavetwins?“Peoplewanttounderstand,”Boomsmasays.ResearcherslikeBoomsmahavesomeideas,especiallyasfraternaltwinbirthsareontheriseinWesterncountries—forinstance,theUnitedStatessawa76%increasefrom1980to2011.Invitrofertilization,forwhichdemandhassurged,ismorelikelytoyieldtwins.Olderwomen,whoarehavingmorechildrenthaninthepast,arealsomorelikelytoreleasemorethanoneegg,increasingtheirchancesofgivingbirthtofraternaltwins.Todefinitivelyunravelthemystery,anewteamledbyHamdiMbarek,amoleculargeneticistatVrijeUniversiteit,combineddatafromalmost2000mothersoffraternaltwinscomingfromdatabasesintheNetherlands,Australia,andMinnesota,andcomparedthemagainstwomenwhohadn’thadtwins,orwomenwhohadhadidenticaltwins.Theresear