  The use of twins designs to address causal questions is becoming increasingly
popular. A standard assumption is that there is no interference between
twins---that is, no twin's exposure has a causal impact on their co-twin's
outcome. However, there may be settings in which this assumption would not
hold, and this would (1) impact the causal interpretation of parameters
obtained by commonly used existing methods; (2) change which effects are of
greatest interest; and (3) impact the conditions under which we may estimate
these effects. We explore these issues, and we derive semi-parametric efficient
estimators for causal effects in the presence of interference between twins.
Using data from the Minnesota Twin Family Study, we apply our estimators to
assess whether twins' consumption of alcohol in early adolescence may have a
causal impact on their co-twins' substance use later in life.
