=+d. My brother bought the season pass. After the summer passed by, my mother said: “I just can’t understand how two kids can turn out so differently. One of them spends all his time during the summer at the swimming pool, while the other barely went at all.” One possible explanation for my mother’s observation is certainly that I am very different from my brother. The other is that we simply faced different circumstances but are actually quite alike. Could the latter be true without large substitution effects?