As the Little Sisters grew older in Rapture, the physical and mental conditioning forced on them had a terrifying aftermath on their bodies. Although they could no longer produce ADAM themselves, their exposure to the substance made them grow taller than most normal girls their age and granted them incredible strength. The mental conditioning given to them by Dr. Yi Suchong, which once tricked them into seeing Rapture as a beautiful utopia, slowly drove them insane and made them highly aggressive.   One more problem with the pubescent girls was that traces of Plasmids found in the ADAM they recycled began to show in the girls' genes, allowing them to use these dangerous powers against anyone who crossed them. As more and more of the Little Sisters began to grow into these feral beasts, Gilbert Alexander decided to use new mental conditioning methods and advanced diving suits to convert the girls into a new form of protector. These 'Big Sisters' (a name chosen by the Little Sisters) would not only protect the younger ones but also retrieve any ADAM stolen from the Rapture Family.