I've been thinking about this a lot recently, and don't have a good answer. Everything that came to mind at first (penis, facial hair, 'Male Answer Syndrome') I discarded, either because a man would still be a man without the feature, or because a woman would still be a woman with the feature.

Does it all come down to "massive amounts of testosterone floating around"? Maybe it does.

Magnanimity.

Good question, and I'm not sure that I'm capable of knowing the answer.

I did the "boy act" for 47 years, and I never understood men. I'm able to describe the kind of Man I tried to be, even the kind of Man I wanted to be, though.

Strong but not brutal. Kind but not overly sentimental. Courageous without being arrogant about it. Protector of those weaker than himself. Manly without being full of Macho BS. Courteous, polite, but unafraid to resist when challenged. Forthright. Chivalrous, never kicking a man who was down, but never giving up when fists were still flying. Indomitable. A quiet achiever.

Succeeding at all this is not required: giving it your best shot is.

Kipling said it best:

IF you can keep your head when all about you
Are losing theirs and blaming it on you,
If you can trust yourself when all men doubt you,
But make allowance for their doubting too;
If you can wait and not be tired by waiting,
Or being lied about, don't deal in lies,
Or being hated, don't give way to hating,
And yet don't look too good, nor talk too wise:

If you can dream - and not make dreams your master;
If you can think - and not make thoughts your aim;
If you can meet with Triumph and Disaster
And treat those two impostors just the same;
If you can bear to hear the truth you've spoken
Twisted by knaves to make a trap for fools,
Or watch the things you gave your life to, broken,
And stoop and build 'em up with worn-out tools:

If you can make one heap of all your winnings
And risk it on one turn of pitch-and-toss,
And lose, and start again at your beginnings
And never breathe a word about your loss;
If you can force your heart and nerve and sinew
To serve your turn long after they are gone,
And so hold on when there is nothing in you
Except the Will which says to them: 'Hold on!'

If you can talk with crowds and keep your virtue,
' Or walk with Kings - nor lose the common touch,
if neither foes nor loving friends can hurt you,
If all men count with you, but none too much;
If you can fill the unforgiving minute
With sixty seconds' worth of distance run,
Yours is the Earth and everything that's in it,
And - which is more - you'll be a Man, my son!

I guess I tried to be the kind of man that I wanted in a husband, or rather, as the father of my children (given the biological realities). The kind of man my father was. The kind of man I hope my son will be.

I flunked masculinity, but I think that I managed to be a decent human being. I've tried to keep as much as I could of the "manly virtues" I'd acquired (to however small a degree). Now that may make me less of a woman - it certainly feeds my insecurities - but I value them more than mere conventional femininity.

I've become more confident now, more sure in myself, and able to express my compassion, and my anger at injustice even more, without being seen as an overbearing bully. I can hug now, and express physically my charity towards my fellow human beings, without it being seen as a sexual advance. I can even allow myself to get angrier, I don't have the testosterone-fuelled capability of physical violence to keep in check. No more beast to keep leashed.

As I said, I flunked masculinity. Things that guys do instinctively, I had to work at. My brain just isn't set up for that, and although I could fake it quite well in some areas, in others you just either have it or you don't.

What makes someone a man? First be a decent human being. That's a pre-requisite. There's many males who have all the rest, things I never did, but not that.

It's interesting to think of the word as opposed to either "boy" or "woman."