Noon, the next day.
The Hagukumi Navi office was a single small room in a multi-tenant building. A few desks, children's picture books, a laptop left open, a mug gone cold. The place where Ms. Mano had spent ten years delivering words to mothers in the middle of the night.
Mio, fresh off an all-nighter, opened her laptop and showed Ms. Mano the screen.
"I'll start with the conclusion." There was no hesitation left in Mio's voice. "The reason the site's 'recommendations' went wrong isn't a bug, or collaborative filtering misreading things, or your fault. Someone poured fake popularity in from outside. It's called data poisoning."
Ms. Mano's eyes went wide.
"Fake... popularity?"
"Yes." Mio lined up two line graphs on the screen. One went jaggedly, irregularly up and down; the other ran high and straight, as if drawn with a ruler. "The top one is a genuinely popular article. When it becomes a topic of conversation, it jumps within a few days, and when people get bored, it sinks right back down. Human interest is this messy and this fickle."
Mio traced the straight lower line with her finger.
"This is the baby carrier in question. For these past few months it's stayed high as if stamped out, and hasn't sunk once. Popularity nobody ever gets bored of doesn't exist in the world of living things. A machine is just leaving it switched on."
Mio opened the list of accounts that had given the baby carrier high stars. All of them had registered recently.
"These few dozen are the culprits. As long as you look at them one at a time, every one of them is a respectable user. Their reviews are inoffensive, and shake them as hard as you like, not a speck of dust comes out. Until you bundle them, you'll never catch them."
Mio pulled the view back to show the screen as a whole flock.
"But bundle them, and a different face comes out."
Mio lined up two graphs. The horizontal axis was the time of day. The vertical axis, how much hands had moved during that time window. A picture bundling a full day's worth of "when" the flock was applauding.
"This is how real customers move," Mio said, pointing to one of the waves. One big peak late at night. Small valleys at midday and in the evening. "In the middle of the night, at feeding and night-crying time, sleep-deprived fingers reach out all at once. During the day their hands are full, so it goes calm. It's jagged, and it smells of life. A human day is this lopsided."
Mio pointed to the line beside it. Morning, noon, the middle of the night, it ran on endlessly flat, at the same height, as if a ruler had been laid against it.
"This is the flock that carried that baby carrier. At three in the morning and at noon, the hands move without a hair of difference. No peaks, no valleys. It's flat to the point of being creepy." Mio's voice dropped. "Humans sleep. This flock doesn't. Even if each one could disguise itself as exactly like the real thing, the bundled 'clock' was the one thing it couldn't hide. The traces of twenty-four hours smoothed out flat and scattered. Machines have no night."
"If you wanted to fake it, you could," Mio added, getting ahead of the question. "Make a peak at night, carve a valley at midday. Easy for one account. But keeping up tens of thousands of accounts' worth of 'daily rhythms,' exactly like the real thing, for months on end, gets expensive. The more a business beats you down with numbers, the more it cuts corners there. That's why it shows when you bundle them."
Ms. Mano's face was turning pale.
"...So these ones were sneaking in pretending to be mothers," she said, her voice trembling. "Mixed in with the real, sleep-deprived mothers."
"Yes."
"Why would anyone do that..."
"Your site is a system that recommends what similar users liked. It's excellent, but it has a weak spot. Pour in a huge number of votes from outside that pretend to be 'similar users,' and you can hijack wholesale who gets recommended what. It's probably some company that wants to sell those unknown products. They freeloaded on ten years' worth of your trust."
Mikoshiba added gently, "Ms. Mano. You didn't do anything wrong. If anything, you were targeted because you were trusted. Burglars go for houses with something inside."
Tears slowly welled up in Ms. Mano's eyes.
"...I was sure I'd messed something up."
"No," Mio denied it at once. "You're the victim."
Mikoshiba folded his arms. "Can't we lock that company out, connection and all? Wouldn't blocking the access sources end it?"
"That was the first thing I tried, but it didn't work." Mio shook her head. "The flock's sources were scattered all over the country, in unrelated households."
"Households?" Ms. Mano knitted her brows.
"'Smart appliances' that have been eaten by viruses. Hijacked security cameras and routers are being used as stepping stones. And free apps, the 'earn pocket money just by walking' kind. Some of those secretly sublet their users' connections. The connections of well-meaning people who installed something for free are the company's cover," Mio went on. "Strike at the access sources and innocent households get caught up too. The culprit is far behind the shield."
Mio switched screens. A list of the rebuilt recommendations. The baby carrier was no longer there. Instead, "lazy-mom baby food" for sloppy mothers had returned near the top.
"I've cleaned up the dirt here. But they'll come again with the same trick. So what I'm leaving you isn't a medicine, it's a system."
Mio brought up a single small button in the corner of the screen. In plain but clear letters: "This might be off."
"I've put a report button in the corner of the articles. A mom who's reading and feels 'this "recommendation" seems weird' only has to press it. No reason or long explanation needed."
"And what happens with that?" Ms. Mano asked.
"You look at it." Mio opened the admin-side screen. The articles that had collected reports were lined up, linked to the accounts that had carried them. "You check them one at a time, and if it's a genuine sales push, you lower the ratings of the accounts that carried it all at once. When the carriers' votes get lighter, the weight of the recommendation naturally sinks too."
Mio looked straight at Ms. Mano.
"I don't leave the final stamp to the machine. If I did, it would mistake your most important regulars for shills and cut them, the way I did yesterday." Mio lowered her eyes slightly. "You, who've watched over mothers for ten years—please decide them one at a time. So that your 'similar' never gets dirtied from outside again."
Ms. Mano looked at the rebuilt recommendations for a long time. Lazy-mom baby food. Cheap undershirts. How to get through night crying. The things she had chosen for ten years for mothers in the middle of the night.
"...This site is the place where I gathered everything I wished I'd had back when I was struggling," Ms. Mano said, still looking at the screen. "When my child wouldn't stop crying in the middle of the night, and I had no one to rely on. A place where people could feel relieved had turned into some stranger's sales pitch. It felt like I was lying to the mothers, and it hurt the whole time."
Ms. Mano finally looked up. Her eyes were wet.
"...Now I won't have to lie anymore. I can hold my head up to those mothers in the middle of the night. Thank you so much."
A few days later, in a corner of Hagukumi Navi, the "This might be off" button began to be pressed, here and there, one at a time. Mothers keeping company with their babies' night crying sent in their sense that something was off, in the gaps between getting them to sleep. Every morning, Ms. Mano and her staff worked through them one by one. Each time something was stamped black, the carriers' votes got lighter. That unknown baby carrier sank toward the bottom of the rankings day by day. Many sleep-deprived hands, one press a day each, brought the site back to where it had been.
Before long, the contract fee was deposited. The amount wasn't large. But in the memo field, a single line from Ms. Mano had been added in meticulous letters: "Thank you for finding the 'real thing' on our site again."
"One more ally," Mikoshiba said, looking at that one line for a long time. "Trust you can't buy with advertising, piling up one at a time."
Mio listened beside him while thinking about something else.
Data had been distorted from outside. Someone had poured in fake behavior and hijacked the judgment. The first "poisoning" she had clearly touched. A system built to protect always has ways it can be abused, too. Mio quietly stored the feel of it somewhere different from the other numbers. Where it connected to, she couldn't see yet.