The studies which show that it is possible to build muscle and rewire the brain at any age.
The many world-class performers who were mediocre at an earlier stage of their development. (Michael Jordan and Tom Brady are obvious examples.)
The 40+ % increase in American support for gay marriage over the past few decades.
The finding that through the course of their lives people become, on average, less open, less extraverted, more agreeable, and (mostly) more conscientious.

The insights of deep canvassing and motivational interviewing. (All persuasion is self-persuasion!)
The end of history illusion illusion: Most people predict they will not change much in the upcoming decade; but when asked about the previous decade, they’ll tell you they’ve changed a lot.
Basically every novel ever.
The approximately 40% of married individuals who wake up one day and realize they are no longer compatible with their spouse.
The fact that ~10% of these divorced couples will end up reconciling and getting back together.
The fundamental attribution error: What we ascribe to personality is often just the power of the situation. (What is life if not a series of new situations?)
The sidewalk poetry of Jane Attanucci:
None of the above is proof that change is inevitable, that human nature is infinitely malleable, or that personal development is entirely under conscious control. However, if you think people are more-or-less stuck the way they are, this list should give you pause. Clearly some individuals are capable of change at least some of the time. This begs the questions: When, how—and why not you?
Your grandmother once said to me “life stage is a compelling force”. Sometimes we change simply because we are in a different place in life.