set.seed(0)p = list()

for(i in 1:1000){

X1 = rnorm(100, 0, 1)

X2 = 0.5*(X1 + rnorm(100, 0, 1))

X3 = 0.5*(X2 + rnorm(100, 0, 1))

growth_condition_differs = 1:50

X1[growth_condition_differs] = X1[growth_condition_differs]*1.5

X3[growth_condition_differs] = X3[growth_condition_differs]*1.5

p[[i]] = coef(summary(lm(X3 ~ X1 + X2)))[2,4]

}

mean(unlist(p)<0.05) # 0.062