Once, when I studied history for a non-living, I became fascinated with emigrants. For a time, I read everything I could find about the people who left for America, about their profound disruption, their strengths, their heroism.

Perhaps it was my prejudice as an American, but in those days I thought very little about the people who stayed behind.

But lately I've been wondering about the difference between those who emigrated and those who didn't. I wonder whether one person remained in Ireland or Italy or Russia because he was content or because he was resigned. I wonder whether another left because she was desperate or because she was hopeful. Was one person more adventurous or another more committed?

In the most desperate moments -- the potato famines, the pogroms -- which took the greater strength of character, endurance or uprooting? Who were the heroes and who were the martyrs? The emigrants or those who remained behind?

I am thinking about this, I'm sure, because many of my friends are turning 40. It began happening three years ago and will, at this rate, continue for at least another five.

Forty is, I observe from not too great a distance, an awkward age. It's an age at which people have histories and options. At 30, they had perhaps less history. At 50, perhaps fewer options.

But at 40, it hangs in the balance. The status quo is weighed against the possible. The person we thought we might be, still challenges the person we are. At 40, many reassess themselves and their circumstances. They try to come to terms with their limits or to break out of them.

I don't want to force the emigrant analogy. For many of our ancestors, the only option was survival and the only feeling was despair. But for a people who now contemplate leaving lives the way their forebears left homelands, the comparison is fair enough.

I think we have as much difficulty knowing what is right or wrong, what is brave or foolish, destructive or adventurous, resigned or realistic when we look at our modern lives.

I know, for example, a writer who turned 40 and decided that he was after all, only a minor talent. He would never be Will Shakespeare, he said, and settled down to a job writing advertising copy. Was he a quitter or a realist? Did he sell himself short or did he find his place?

I know a woman who turned 40 and decided after years of marital indecision and separation that this was her husband, her life -- this was "it." It was, she said, okay. Did she settle for less or did she settle down?

On the other hand, there is the 40-year-old man who decided that his history was rot and, to change his future, he left much of his past. How does one judge his action? By just how intolerable his circumstances were? By what his future brings? Did it take more guts to leave than to stay?

My mind curves around all these things like a question mark. It is hard to hold still long enough to make sense of them. I wonder again if we can only judge actions by motivations. Is the person who endures masochistic or virtuous? Is the person who takes off sane or irresponsible?

Introspection is as painful as it is inevitable. At some time in our lives, especially our mid-lives, we take stock like a department store. We face ourselves and our circumstances. We try to be reasonable about our lives, compare what we have (and could lose) with what we might have. We talk about the necessity of compromising and the fear of compromising ourselves. Our satisfactions battle our fantasies.

We try to make rational judgments, I suppose, about protecting the status quo or chainging it. But in the end, some of us emigrate to our new worlds and some of us stay with the familiar. And the future historians will have trouble understanding the differences.