If identical twins are more alike than fraternal twins in a particular trait, the increased similarity must be due to: similar environments for identical and fraternal twins.
(identical twins; are; more alike than fraternal twins in a particular trait) (the increased similarity; must be; due to: similar environments for identical and fraternal twins)
identical twins the increased similarity
are must be
more alike than fraternal twins in a particular trait
due to: similar environments for identical and fraternal twins