Human civilization had a lot of branches, not so long ago. Even into the twenty-first century, a few isolated tribes had barely developed stone tools. Some settled down with agriculture. Others weren't content until they had ended nature itself, still others until they'd built cities in space.

We all rested eventually, though. Each new technology trampled lesser ones, climbed to some complacent asymptote, and stopped—until my own mother packed herself away like a larva in honeycomb, softened by machinery, robbed of incentive by her own contentment.

But history never said that everyone had to stop where we did. It only suggested that those who had stopped no longer struggled for existence. There could be other, more hellish worlds where the best Human technology would crumble, where the environment was still the enemy, where the only survivors were those who fought back with sharper tools and stronger empires. The threats contained in those environments would not be simple ones. Harsh weather and natural disasters either kill you or they don't, and once conquered—or adapted to— they lose their relevance. No, the only environmental factors that continued to matter were those that fought back, that countered new strategies with newer ones, that forced their enemies to scale ever-greater heights just to stay alive. Ultimately, the only enemy that mattered was an intelligent one.

And if the best toys do end up in the hands of those who've never forgotten that life itself is an act of war against intelligent opponents, what does that say about a race whose machines travel between the stars?

The argument was straightforward enough. It might even have been enough to carry the Historians to victory—if such debates were ever settled on the basic of logic, and if a bored population hadn't already awarded the game to Fermi on points. But the Historian paradigm was just too ugly, too Darwinian, for most people, and besides, no one really cared any more. Not even the Cassidy Survey's late-breaking discoveries changed much. So what if some dirtball at Ursae Majoris Eridani had an oxygen atmosphere? It was forty-three lightyears away, and it wasn't talking; and if you wanted flying chandeliers and alien messiahs, you could build them to order in Heaven. If you wanted testosterone and target practice you could choose an afterlife chock-full of nasty alien monsters with really bad aim. If the mere thought of an alien intelligence threatened your worldview, you could explore a virtual galaxy of empty real estate, ripe and waiting for any God-fearing earthly pilgrims who chanced by.

It was all there, just the other side of a fifteen-minute splice job and a cervical socket. Why endure the cramped and smelly confines of real-life space travel to go visit pond scum on Europa?

And so, inevitably, a fourth Tribe arose, a Heavenly host that triumphed over all: the Tribe that Just Didn't Give A Shit. They didn't know what to do when the Fireflies showed up.

So they sent us, and—in belated honor of the Historian mantra—they sent along a warrior, just in case. It was doubtful in the extreme that any child of Earth would be a match for a race with interstellar technology, should they prove unfriendly. Still, I could tell that Bates' presence was a comfort, to the Human members of the crew at least. If you have to go up unarmed against an angry T-rex with a four-digit IQ, it can't hurt to have a trained combat specialist at your side.

At the very least, she might be able to fashion a pointy stick from the branch of some convenient tree.

"I swear, if the aliens end up eating the lot of us, we'll have the Church of Game Theory to thank for it," Sascha said.

She was grabbing a brick of couscous from the galley. I was there for the caffeine. We were more or less alone; the rest of the crew was strewn from dome to Fab.

"Linguists don't use it?" I knew some that did.

"We don't." And the others are hacks. "Thing about game theory is, it assumes rational self-interest among the players. And people just aren't rational."

"It used to assume that," I allowed. "These days they factor in the social neurology."

"Human social neurology." She bit a corner off her brick, spoke around a mouthful of semolina. "That's what game theory's good for. Rational players, or human ones. And let me take a wild stab here and wonder if either of those is gonna apply to that." She waved her hand at some archetypal alien lurking past the bulkhead.

"It's got its limitations," I admitted. "I guess you use the tools you can lay your hands on."

Sascha snorted. "So if you couldn't get your hands on a proper set of blueprints, you'd base your dream home on a book of dirty limericks."

"Maybe not." And then, a bit defensive in spite of myself, I added, "I've found it useful, though. In areas you might not expect it to be."

"Yeah? Name one."

"Birthdays," I said, and immediately wished I hadn't.

Sascha stopped chewing. Something behind her eyes flickered, almost strobed, as if her other selves were pricking up their ears.

"Go on," she said, and I could feel the whole Gang listening in.

"It's nothing, really. Just an example."

"So. Tell us." Sascha cocked James' head at me.

I shrugged. No point making a big thing out of it. "Well, according to game theory, you should never tell anyone when your birthday is."

"I don't follow."

"It's a lose-lose proposition. There's no winning strategy."

"What do you mean, strategy? It's a birthday."

Chelsea had said exactly the same thing when I'd tried to explain it to her. Look, I'd said, say you tell everyone when it is and nothing happens. It's kind of a slap in the face.

Or suppose they throw you a party, Chelsea had replied.

Then you don't know whether they're doing it sincerely, or if your earlier interaction just guilted them into observing an occasion they'd rather have ignored. But if you don'ttell anyone, and nobody commemorates the event, there's no reason to feel badly because after all, nobody knew. And if someone doesbuy you a drink then you know it's sincere because nobody would go to all the trouble of finding out when your birthday is— and then celebrating it—if they didn't honestly like you.

Of course, the Gang was more up to speed on such things. I didn't have to explain it verbally: I could just grab a piece of ConSensus and plot out the payoff matrix, Tell/Don't Tell along the columns, Celebrated/Not Celebrated along the rows, the unassailable black-and-white logic of cost and benefit in the squares themselves. The math was irrefutable: the one winning strategy was concealment. Only fools revealed their birthdays.

Sascha looked at me. "You ever show this to anyone else?"

"Sure. My girlfriend."

Her eyebrows lifted. "You had a girlfriend? A real one?"

I nodded. "Once."

"I mean after you showed this to her."

"Well, yes."

"Uh huh." Her eyes wandered back to the payoff matrix. "Just curious, Siri. How did she react?"

"She didn't, really. Not at first. Then—well, she laughed."

"Better woman than me." Sascha shook her head. "I'd have dumped you on the spot."

My nightly constitutional up the spine: glorious dreamy flight along a single degree of freedom. I sailed through hatches and corridors, threw my arms wide and spun in the gentle cyclonic breezes of the drum. Bates ran circles around me, bouncing her ball against bins and bulkheads, stretching to field each curving rebound in the torqued pseudograv. The toy ricocheted off a stairwell and out of reach as I passed; the major's curses followed me through the needle's eye from crypt to bridge.