Ob jec tive: To re view re cent de vel op ments in the study of non shared en vi ron ment; that is, the en vi r on mental in flu ences Con clu sions: De spite the dif fi cul ties en coun tered in iden ti fy ing spe cific sources of non shared en vi ron ment, the fact re mains that most en vi ron mental vari ance af fect ing the de vel op ment of psy cho logi cal di men sions and psy chi at ric dis or ders is not shared by chil dren grow ing up in the same fam ily. More re search and the ory are needed to ex plain why such sib lings are so dif fer ent.Chance, in the sense of idio syn cratic ex pe ri ences, also needs to be con sid ered. O ne of the most im por tant dis cov er ies aris ing from behavioural-genetic re search con cerns nur ture rather than na ture. Behavioural-genetic re search pro vides the best avail able evi dence for the im por tance of en vi ron mental in fluences, but it shows that the en vi ron ment works in a sur pris ing way. Al though so ciali za tion theo ries as sume that the en viron ment of the fam ily unit in flu ences psy cho logi cal de vel opment, with the re sult that shared ex pe ri ences lead to simi lari ties among sib lings, behavioural-genetics re search con sis tently in di cates that chil dren grow ing up in the same fam ily do not share ef fec tive en vi ron mental in flu ences. It is not shared ex pe ri ences, but shared ge net ics, that make siblings re sem ble one an other. In deed, with re gard to psy chologi cal de vel op ment, en vi ron ment makes sib lings no more simi lar to one an other than to chil dren picked at ran dom from the gen eral popu la tion. This phe nome non-the fact that effec tive en vi ron ments are not shared-has been termed "nonshared en vi ron ment."Our goal is to re view re cent de vel op ments in this field and point to new di rec tions for re search. We be gin with a brief over view of the phe nome non of non shared en vi ron ment. The PhenomenonNon shared en vi ron ment lay hid den within behaviouralgenetic stud ies since such stud ies be gan 80 years ago. Twin stud ies and adop tion stud ies were de signed to dis en tan gle the ele ments of na ture and nur ture in fa mil ial re sem blance. For ex am ple, schizo phre nia runs in fami lies, and en vi ron men talists had as sumed that it does so for en vi ron mental rea sons: fam ily mem bers share the same schizo phre no genic en vi ronment. Twin stud ies and adop tion stud ies test this as sump tion by study ing fam ily mem bers who share na ture or nur ture to vary ing de grees. Al though most rele vant evi dence has come