All you need is love - And a little rationality

A Virtual Insanity from The Rationality of Love

Love is often portrayed as the one place where economics goes to sleep. We compare prices for a phone, haggle over a used motorbike, and weigh every rupiah of our monthly allowance, yet when it comes to the person we love, the calculator is supposed to go quiet. In this view, love is a beautiful madness, and any economist who tries to model it has simply missed the point.

Hatta’s last year Kajipost post tells this tale through Adam and Jane. Adam overspends on a first date and hides his temper, his poor hygiene and an outdated profile photo, winning over a Jane who sees a thoughtful, book-loving animal lover. As the dates pile up, the act wears off, and in the end Jane discovers that he has been chatting with and meeting his ex. She stays anyway. Becker’s marriage market, the post argues, can explain the early dates but not the hundred and first, and so love must be irrational (Hatta, 2025). Such is the tale of our hearts. Yet Jane’s choice may tell a different story. Rather than abandoning reason, she may be doing exactly what the theory asks of her, only with a utility function that holds more than herself that is an endogenous one. So if economics was supposed to break at the door of love, why does a model written by the very same economist predict that she stays? And if staying is predicted, why does it still look like virtual insanity from the outside? The truth may be less mystical than a madness, and a little more dynamic than a ruler.

A Harsh Truth from the 101st Date

“Love, by its very nature, is irrational.”

• Hatta (Kanopi FEB UI, 2025)

By the hundredth date, the restaurant Adam could not afford had long been replaced by modest cafés, the special-occasion perfume stayed in the cabinet, and the voice that once charmed Jane was now raised at her whenever things got difficult. Then came the betrayal. Any reader would tell her to leave. She stays.

Hatta’s work begins with Gary Becker’s. In his 1973 paper, Becker treats marriage as a partnership in which two people jointly produce and consume household commodities, and choose partners according to their traits (Becker, 1973). From this angle, Jane’s affection is easy to explain when she saw kindness, humor, and thoughtfulness, and the presence of Adam itself maximized her utility given her alternatives. That works for the second, third, even fourth date. It does not work for the 101st, after the flaws and the betrayal are known, because a purely rational agent would leave. The conclusion is that people run on gut feeling more than calculation, the fast System 1 of Kahneman (2011) from his Thinking, Fast and Slow, and that love is what keeps us from becoming what Sen once called a “social moron”.

It is a persuasive story, and partly a correct one. The cold, self-interested Homo economicus is a bad description of real people. Staying, forgiving, and sacrificing are the real puzzle. Furthermore, Becker’s 1973 paper alone cannot explain why Jane stays after the mask slips. It deserves credit for choosing the right puzzle.

The harsh truth of the verdict is that the word “irrational” is doing too much work. In Hatta’s work, it stands for three different things: inconsistent preferences, wrong beliefs, and unselfish preferences. Only the first violates rationality as standard theory defines it, which asks for consistency, not selfishness. A parent who skips dinner so the child can eat is not confused about their own preferences; the child’s meal is simply on the parent’s own menu. Its examples are mostly of the second and third kind. If unselfish meant irrational, then every parent, every blood donor and every friend who ever helped you move house would be irrational, which is a rather strange theory of rationality. Of course, the hundred-and-first date is only a puzzle if Jane’s utility depends on hers alone. If.

Revelations from the Second Paper

“Look not every man on his own things, but every man also on the things of others.”

• Philippians 2:4 (KJV)

The 1973 paper on which the argument builds sets love aside. Its sequel, published in 1974, opens by admitting as much: Part I had ignored love, the cause of marriage that American culture so glorifies, and Part II sets out to bring it in (Becker, 1974a). Becker’s reasoning is simple. Since loving someone usually involves caring about what happens to them, the model should let a person care. Caring implies sharing, and when it is full and mutual the two share equally, which fits the popular belief that persons in love “share” (Becker, 1974a). The formal backbone comes from a companion paper on social interactions, which places the utility of another person inside the utility function of the one who cares (Becker, 1974b).

Two things are therefore added to an ordinary utility function. The first is presence, where the time spent with our loved one is a good in its own right, and Becker (1973) already counts companionship and love among the commodities a household produces and consumes. The second is caring, where a share of the loved one’s utility enters our own. Think of two speakers facing each other. Her good day lifts his, and consequently lifts hers again. Nobody is confused; the signal is simply shared. In a simplified version, with a caring weight α for Jane and β for Adam, each person’s utility is what they get from their own life (consumption and time together) plus a share of their partner’s:

Because each utility contains the other, the two have to be solved together, and that is all that “endogenous” means here:

When both people care, the factor 1 / (1 − αβ) is larger than one, so each person’s wellbeing is amplified by the other’s. The model also has to keep αβ below one, or the loop would blow up, which is another way of saying that two utilities have fully merged into one.

The results are not mystical. The more Jane cares, the more she tolerates, since Adam’s flaws cost her through her own utility while leaving would cost her through his. As the dates pile up, two things move at once: what Jane learns about Adam, which pulls the value of staying down, and how much she cares, which plausibly grows with shared history. The verdict counts only the first. The mask slipping is not irrationality but information. And the model helps explain her decision if she were to decide on leaving Adam: whether what she gains from leaving, net of the cost of switching, outweighs the weight she places on Adam’s loss. Betrayal lowers the value of staying and may lower α itself, and better options raise the value of leaving. “Love is irrational” fits both a Jane who stays and a Jane who leaves, so it predicts neither.

A fair objection is that Adam’s bad temper, poor hygiene and cheating do more than reveal information, because they directly damage Jane’s life. They do, and the model takes them in. A bad temper and a bad smell lower what Jane gets from a day with Adam, which is the first part of her utility, and a betrayal can wound her sense of self-worth and may lower α itself. Each of these makes staying cost more, so the model predicts that Jane leaves sooner, and a Jane who stays anyway simply has not crossed her threshold yet. Her utility is not ignoring the flaws; it is weighing them against what Adam’s wellbeing, their shared history and the cost of starting over are worth to her. She is not settling for less in any way the model cannot see. She is trading less of one thing for more of another, which is all a rational choice has ever been.

Nor is it a stretch to treat unselfish choices as ordinary preferences. In an experiment where people split money with an anonymous stranger, Andreoni and Miller (2002) found that only about a quarter of subjects were pure money maximizers, while the rest showed varying degrees of altruism, and they concluded that people hold a consistent preference for it. Unselfish, yes. Inconsistent, no. Love breeds care, care breeds tolerance, and tolerance, quite rationally, breeds a 101st date.

The Limits of a Rational Heart 

Unfortunately, a model that fits everything is a dangerous thing to own. Put anything inside a utility function and it will explain anything: staying, leaving, forgiving, sulking. Picture a regression with an R² of one, handed down by God himself, in which every choice Jane ever makes is fitted exactly. It would describe her flawlessly and teach us nothing, much like a student who memorizes last year’s exam and has learned nothing about this year’s. A theory earns its keep only through restrictions that could turn out to be wrong. Ours makes a few, caring is bounded, more caring means more tolerance, and betrayal and better options push toward leaving. They are modest, and data can embarrass them. They are also the reason a rough model that gets the direction right is worth more than a perfect fit that can never be wrong.

This is also where the sharpest objection to our model lives. The caring weights α and β are different for every person on earth, and they may shift with the day and the environment. Since nobody can know the number, the whole thing gets filed under irrationality. But not knowing a number is a fact about the economist, not about Jane. Rationality asks whether her choices hang together, not whether an outsider can read her coefficient. And economics has never needed the number. Nobody knows the exact demand curve of any single shopper, which differs from person to person and from one payday to the next, yet we are confident that a higher price lowers what they buy. What we need is the direction. Is Adam’s wellbeing a positive weight in Jane’s utility (α > 0), no weight at all (indifference), or a negative one (spite)? The sign alone is enough to predict that more love means more tolerance. Physicists did not see antimatter first either: an equation told them it had to exist and which way it would behave, long before a detector caught one. One honest limit remains. If α swung wildly from day to day for no reason, the model would explain nothing. My claim is that it moves for reasons we can name, such as shared time or in some cases betrayal, and that is what makes it testable in a sense.

Even then, the model has an edge, and Sen is the one who marked it. In Rational Fools, Sen (1977) separates sympathy, where another person’s welfare enters one’s own, from commitment, where one acts on a principle or a duty even when it leaves them worse off. Sympathy sits coherently inside our model. Commitment, which drives a wedge between what we choose and what makes us better off, does not. This is the same Sen whose line about the social moron is so often quoted against economics, yet his complaint was aimed at the purely economic man, not at rationality itself. His remedy was a richer account of rational choice, not a farewell to it. If Jane stays out of sympathy, she is inside the model. If she stays out of duty regardless of her own welfare, she has walked past it, and that is where I admit where the limit of this model lies. 

There are other bruises worth admitting here, love may distort what we believe about our partner, making us see them as the funniest and most fascinating person alive. That is a real bias that is often present, but beliefs are not preference, and the caring structure above does not depend on it. People also stay out of fear, habit and sunk costs, which are switching costs rather than love, and the model keeps the two apart. Finally, explaining why Jane stays is not saying that she should (again its positive folks). A theory of why people stay does not turn a harmful relationship into a good one, and that threshold exists precisely so that it can be crossed in some cases.

The Madness We Chose

Perhaps, then, love was never an exception to rational choice. It was a variable that often economic textbooks forgot to include. Jane’s staying is not an error term but a prediction: someone whose utility holds Adam’s wellbeing and presence will tolerate a great deal before the scale finally tips. It looks like madness from the outside, the virtual kind, produced by a rational agent whose utility function is simply larger than herself. Love is indeed what makes us human, and the rational chooser of the textbook was never meant to be its opposite. Homo economicus and the social creature can both be true, as long as Homo economicus means a consistent chooser rather than a selfish one: the first describes how we choose, the second describes what we choose among, and that includes the people we love. The model’s only addition is that love makes our utility function bigger than we are.

The theory will never tell us the date Jane leaves, and it will never be God’s regression with an R² of one. But it gets the direction right, and there's a dead end where it ends, which is more than the verdict of madness can say. So when Jane stays on the hundred-and-first date, is it virtual insanity? Or is it simply a utility function with someone else in it? But hey, that's just a theory—a game theory! 

Lastly, I will end this, In the words of Ursula K. Le Guin “When I take you to the Valley, you’ll see the blue hills on the left and the blue hills on the right, the rainbow and the vineyards under the rainbow late in the rainy season, and maybe you’ll say, “There it is, that’s it!” But I’ll say. “A little farther.” 

We’ll go on, I hope, and you’ll see the roofs of the little towns and the hillsides yellow with wild oats, a buzzard soaring and a woman singing by the shadows of a creek in the dry season, and maybe you’ll say, “Let’s stop here, this is it!” But I’ll say, “A little farther yet.”

We’ll go on, and you’ll hear the quail calling on the mountain by the springs of the river, and looking back you’ll see the river running downward through the wild hills behind, below, and you’ll say, “Isn’t that the Valley?” And all I will be able to say is “Drink this water of the spring, rest here awhile, we have a long way yet to go and I can’t go without you.

REFERENCES

Andreoni, J., & Miller, J. (2002). Giving according to GARP: An experimental test of the consistency of preferences for altruism. Econometrica, 70(2), 737–753.

Becker, G. S. (1973). A theory of marriage: Part I. Journal of Political Economy, 81(4), 813–846. https://www.nber.org/chapters/c2970.pdf

Becker, G. S. (1974a). A theory of marriage: Part II. Journal of Political Economy, 82(2), S11–S26. https://www.nber.org/chapters/c3681.pdf

Becker, G. S. (1974b). A theory of social interactions. Journal of Political Economy, 82(6), 1063–1093.

Kahneman, D. (2011). Thinking, fast and slow. Farrar, Straus and Giroux.

Hatta, R.H. (2025, June 20). All you need is love—and a little irrationality. https://www.kanopifebui.com/post/all-you-need-is-love-and-a-little-irrationality

Sen, A. K. (1977). Rational fools: A critique of the behavioural foundations of economic theory. Philosophy & Public Affairs, 6(4), 317–344.

The Holy Bible, King James Version. (n.d.). Philippians 2:4.

Ursula K. Le Guin (1985), Always Coming Home.