The interaction between zygosity and co-twin’s FTND score was statistically significant; critically, the direction of the interaction is such that an MZ co-twin’s FTND score was more predictive of nicotine withdrawal-induced depressive symptoms than was a DZ selleckchem Veliparib co-twin’s FTND score. Equation 5. As noted in the Methods section, we also conducted regressions that replaced co-twin’s MD with co-twin’s mean N-score (Equations 5 and 6) since neuroticism is significantly correlated with both GAD and MD and as a continuous variable is more statistically powerful. In Equation 5, which included an interaction between zygosity and co-twin’s neuroticism, increased age, being female, and higher co-twin FTND scores were significantly associated with depressive symptoms.

The main effect of co-twin’s neuroticism was not a significant predictor of withdrawal-induced depressive symptoms in Equation 5 nor was the interaction between that term and zygosity. Equation 6. Equation 6 was similar to Equation 5 but replaced the interaction between co-twin’s neuroticism and zygosity with an interaction between co-twin’s FTND score and zygosity. In this equation, increased age, being female, and being a member of DZ twin pair were significantly associated with more pronounced symptoms of depression. In addition, the main effect of co-twin’s neuroticism was significantly associated with outcome (with higher levels of neuroticism conferring increased risk). Similar to Equation 2, the interaction between co-twin’s FTND and zygosity was significantly associated with outcome, with MZ twins�� FTND scores more predictive of outcome than DZ twins�� FTND scores.

When all feasible predictors and interaction terms are included in the model, only the sex and zygosity �� co-twin’s FTND terms were significantly associated with outcome (data not shown). Withdrawal-Induced Anxiety Symptoms Equation 3. Only sex and co-twin’s FTND score significantly predicted withdrawal-induced anxiety when testing for an interaction between zygosity and co-twin’s GAD, with females endorsing higher levels of depressive symptoms and higher co-twin’s FTND scores conferring greater risk; Batimastat age, co-twin’s GAD, and the interaction term of zygosity �� co-twin’s GAD were not statistically significant predictors. Equation 4. When testing for an interaction between zygosity and co-twin’s FTND score, sex, zygosity, co-twin’s FTND score, and the interaction term zygosity �� co-twin’s FTND score were all significantly associated with nicotine withdrawal-induced anxiety. As with previous equations, being female, being a member of a DZ twin pair, and having a co-twin with a high FTND score conferred increased risk of symptoms of anxiety.