Under either of these explanations, what makes one conservative justice more likely than another to defect? One obvious variable is how genuinely conservative, as a matter of judicial philosophy, the justice is. But I’d suggest that factors of character and personality also come into play. A justice who likes to be liked, or who desires to be admired, or who is eager to go along to get along (what I’ll call Type 1) is far more susceptible to stray than a justice who is inner-directed and anchored, who doesn’t care about being popular or who is even contrary or cantankerous (Type 2). Antonin Scalia, Clarence Thomas, and Samuel Alito are all Type 2.
One unfortunate reality is that the process of selecting and confirming Supreme Court justices strongly favors Type 1 conservatives. The nomination and confirmation processes are intensely political, and the qualities of Type 1 conservatives correlate strongly with qualities that are politically appealing and salient. Advisers to the president want a Supreme Court nomination to be a short-term political victory, and same-party senators strongly prefer a light lift. A Type 1 conservative is going to be much more adept than a Type 2 conservative at charming senators, trotting out a list of liberal friends and admirers, and neutralizing a leftist media—and thus at winning the support of the president and his advisers for the nomination in the first place.