I follow you on Twitter, and I was disgusted to see your tweet about marriage, "No, humans aren't naturally monogamous -- which is why people say relationships 'take work,' while you never hear anybody talking about what a coal mine an affair can be." If a person finds fidelity so challenging, they should stay single.

--Ethical Married Person

Reality has this bad habit of being kind of a bummer. So, sure, that person you married all those years ago still has the capacity to surprise you with crazy new positions in bed -- but typically they're yogi-like contortions they use to pick dead skin off the bottoms of their feet.

That line you quote, "relationships 'take work,' while you never hear ... what a coal mine an affair can be," is actually from one of my old columns. I tweeted it along with this advice: "Don't just assume you & romantic partner (will) stay monogamous. Maybe discuss how, exactly, you'll go about that."

From where I sit -- opening lots of letters and email from cheaters and the cheated upon -- this is simply good, practical marriage- (and relationship-) preserving advice. But from some of the responses on Twitter, you'd think I'd suggested braising the family dog and serving him on a bed of greens with a "tennis ball" of candied yams.

Though some men and women on Twitter merely questioned my take, interestingly, the enraged responses (ranging from impersonally rabid to denigratingly hateful) came entirely from men. Granted, this may just have been due to chance (who was shirking work on Twitter just then), or it may reflect research on sex differences that suggests men tend to be more comfortable engaging in direct conflict.

However, though evolutionary psychologist David Buss, among others, finds that both men and women are deeply upset by infidelity -- or the mere prospect of it -- there seems to be a sex difference in who is more likely to go absolutely berserko over it. Buss, looking out over the anthropological literature, observes: "In cultures the world over, men find the thought of their partner having sexual intercourse with other men intolerable. Suspicion or detection of infidelity causes many men to lash out in furious anger rarely seen in other contexts."

Evolutionary psychologists have speculated that the fierceness of male sexual jealousy may be an evolved adaptation to combat the uniquely male problem of "paternity uncertainty" -- basically the "who actually is your daddy?" question. A woman, of course, knows that the tiny human who's spent a good part of nine months sucker-punching her in the gut is hers. However, our male ancestors lacked access to 23andMe mail-in DNA tests. So male emotions seem to have evolved to act as an alarm system, goading men to protect themselves (like with a scary expression of anger to forewarn their partner), lest they be snookered into raising another man's child.

The problem with enraged response is that it kicks our brain into energy conservation mode -- shunting blood flow away from our higher-reasoning department and toward our arms and legs and organs needed for "fight or flight." So the mere mention of cheating -- even coupled with suggestions for how to prevent it -- kills any possibility of reasoned thinking. In our dumbed-down enraged state, all we've got is the knee-jerk response: "I am so totally moral, and so is my wife, and anyone who needs to discuss how they'll stay monogamous is the Whore of Babylon!"

Unfortunately, aggressive denial of reality is particularly unhelpful for infidelity prevention. It's especially unhelpful when it's coupled with feelings of moral superiority. Organizational behaviorist Dolly Chugh and her colleagues find that people's view of themselves as "moral, competent, and deserving ... obstructs their ability" to make ethical decisions under pressure.

So, as the late infidelity researcher Peggy Vaughan advised, "a couple's best hope for monogamy lies in rejecting the idea that they can assume monogamy without discussing the issue." They should instead admit that "attractions to others are likely ... no matter how much they love each other" and "engage in ongoing honest communication about the reality of the temptations and how to avoid the consequences of acting on those temptations."

For example: What's the plan if, say, marital sex gets a little sparse? If the marriage hits a rough patch? If that hot co-worker starts hitting on you when you're drunk and a little unhappy while on a business trip?

Maybe it seems depressing to discuss this stuff. However, a wedding ring is not an electrified fence. Accepting that is probably your best bet for avoiding emotional devastation and divorce when, 25 years in, a "jug of wine, a loaf of bread, and thou" still keeps the old spark alive in bed -- but only when supplemented with a well-charged cordless cattle prod.

I'm a married lesbian in my 50s. I blew up my happy marriage by having an affair with somebody I didn't love and wasn't even that attracted to. Now my wife, whom I love very much, is divorcing me. Why did I cheat on her? I don't understand my own behavior.

--Lost

There are those special people you meet who end up changing your life -- though ideally not from happily married person to lonely middle-aged divorcee living in a mildewy studio.

There's a widespread assumption that "a happy marriage is insurance against infidelity," explained the late infidelity researcher Shirley Glass. Even she used to assume that. But, her research (and that of subsequent researchers) finds that even happily married people end up cheating -- for a variety of reasons. Sometimes they want better sex or even just different sex. Sometimes they want an ego shine. And sometimes they feel something's missing within them. But soul-searching is emotionally grubby, tedious work, so they first look for that missing something in the nearest hot person's underpants.

It seems inexplicable (and borderline crazy) that you risked everything you care about for somebody you find kind of meh -- until you look at this through the lens of "bounded rationality." And before anybody takes a lighter to hay on a pitchfork they plan to chase me with, I'm simply offering a possible explanation for such baffling behavior; I'm not excusing cheating.

"Bounded rationality" is the late Nobel Prize-winning cognitive scientist Herbert Simon's term for the constraints on our ability to make truly reasoned, rational decisions. These decision-making constraints include having a limited time to make a choice and limited cognitive ability, which keeps us from seeing the whole picture, with its rainbow of repercussions.

We can end up engaging in what psychologists call "framing," a sort of selecta-vision in which we make decisions based on whichever part of the picture happens to be in mental focus at the time. (Of course, we're more likely to focus on how fun it would be to have a little strange than how strange it would be to end up exiled to a motel when the wife finds out.)

For some people, behavior from their spouse that suggests "Ha-ha...crossed my fingers during that vows thing!" is simply a deal breaker. But say your wife still loves you and is mainly leaving because she feels she can't trust you. (A partner who inexplicably cheats is a partner there's no stopping from inexplicably cheating again.)

If you can explain -- though not excuse! -- your thinking (or nonthink) at the time, maybe your wife will agree to try couples therapy, at least for a few months. Bounded rationality aside, I suspect you're unlikely to cheat again, and especially not on what I call "The ER Model" for bad decisions: patients muttering, "This isn't how I thought the night would end" -- just before the doctor extracts the light saber-toting action figure from a place where, no, the sun does not shine but supplemental illumination is generally unnecessary.

How long does it take to get over someone? One friend said it takes half as long as you were together, and another said it takes twice that time.

--Recently Dumped

Sometimes it takes a while to let go, but sometimes you're so ready that you'd chase the person off your porch with a shotgun (if you had a porch or a shotgun and weren't afraid of doing time on a weapons charge).

Your friends, with their precise breakup timetables, are confusing emotional recovery with mass transit. The reality is, people vary -- like in how naturally resilient they are -- and so do relationships. (Some are long over before they're formally retired.)

Sadness after a breakup can feel like the pointless adult version of getting grounded indefinitely. However, as I've written in previous columns, psychiatrist and evolutionary psychologist Randolph Nesse explains that sadness appears to be "adaptive" -- meaning that it has useful functions. For example, the "disengagement" from motivation that accompanies sadness gives us time to process what happened, possibly helping us learn from our mistakes instead of inviting them back in for an eggnog.

Accordingly, a way to heal emotionally is to find meaning within your mistakes -- figuring out what you might have seen or done differently, which tells you what you should probably do differently in the future. In other words, think of the sadness holding you down not as your hostage-taker but as your helper. Deliberately using it that way might even help you curb the impatience that leads some to start dating before they're actually ready. Sure, on a first date, it's good to give a guy the sense that you're passionate and emotionally present, but probably not by sobbing uncontrollably when he asks whether you want a latte.

I'm a woman, and I recently made a new professional connection -- a man who's excited about my work. We're planning on doing a big important project together. I'm worried that he's interested in me romantically (based on a few things he's said). I'm not interested in him in that way. What's the right thing to say to get that across?

--All Business

It's tempting to get everything out in the open right away: "I've run the numbers on your chances of having sex with me, and they're pretty close to the odds of your being crushed to death by a middle-aged dentist falling out of the sky."

Informing a guy pronto that you aren't romantically interested in him -- though in somewhat kinder language -- would be the right thing to do if he were just some persistent Tinder date you wanted to unload forever. But you're hoping to have a continuing business relationship with this guy. So even if it were wildly obvious that he has the hots for you, the last thing you should do is mention that particular elephant in the room (not even while you're pole-vaulting over steaming mountain ranges of elephant dung).

Cognitive psychologist and linguist Steven Pinker points out that "most social interaction" involves some conflicting goals -- for example, when only one of two people is interested in ending the evening in the tool shed/sex dungeon. (Yes, sometimes the nightcap is a rubber hood.)

Pinker explains that "indirect speech" -- not saying exactly what you think or want -- is a way two people can maintain their relationship as it is (even when both suspect or are pretty sure that their desired outcomes are in sharp conflict). The sometimes tiny measure of ambiguity -- uncertainty about another person's goals -- that is fostered by indirect speech does a big job. It allows the person who wants something the other doesn't to save face, enabling the two to preserve their common ground.

So, your refraining from telling the guy that you aren't interested (in so many words) allows him to cling to the ego-preserving possibility that you might be. If he goes direct on you -- tells you he wants to sex up your business relationship -- that's when you likewise get explicit: Tell him straight out that you want to keep things strictly professional. However, this may not be necessary if you act in ways that say "just business!" Avoid going flirty in communicating with him, and schedule meetings for the utterly unsexiest times and places possible. Nobody ends up doing the walk of shame because they had seconds on biscotti and one too many double espressos.

There's always been an attraction between this guy and me. I've been thinking of testing the waters with him romantically, but he recently mentioned that he freaks out when women cry. He says he just has no idea what to do. Well, I'm an emotional person -- generally happy but also a big crier. Are we a bad match, or could I teach him to soothe me?

--Waterworks

Most men are comfortable dealing with any leaky item -- as long as it can be fixed with an adjustable wrench and a Phillips screwdriver.

If there's a decoder ring for human emotion, it's the female brain. Psychologist Simon Baron-Cohen finds that men, generally speaking, just aren't as good as women at what's called "theory of mind" -- the ability to "infer what other people might be thinking or intending." He explains that women, from childhood on, tend to be the "empathizers" of the species, driven to identify others' "emotions and thoughts, and to respond with the appropriate emotions" (say, by hugging a teary-eyed person instead of treating them like a statue weeping blood).

In contrast with female "empathizers," Baron-Cohen describes men as the "systematizers" of the species. This is a fancy way of saying they're engineering-focused -- driven, from a young age, to identify how inanimate stuff works and "derive the underlying rules that govern the behavior of a system." However, these are "reliable" rules, like the law of gravity -- "What goes up must come down" -- nothing helpful for fathoming what the girlfriend's got swirling around in her head when she suddenly goes all funeralface.

Typically, women believe "If he loved me, he'd figure it out." Um, no. Not here in realityland. Assume most heterosexual men are sucky at emotional tea leaf reading. When you're in boohooville (or on your way), tell a man what you're feeling and how he could help -- for example, by just listening and rubbing your back. In time, this may help him avoid reacting to the welling of that very first tear by diving behind the couch and yelling, "Incoming! One o'clock! Alpha team, flank left!"

I saw this gorgeous girl at the coffeehouse at the mall two months ago. It was totally love at first sight. I keep hanging out there hoping to see her again. Am I nuts, or does love at first sight really exist?

--Smitten

It's so special when a man tells a woman he's deeply in love with her -- except when her response is "Excuse me, but have we met?"

Love at first sight sounds so romantic. There are those couples who claim they had it -- causing mass nausea at dinner parties when they look into each other's eyes and announce, "From the moment we saw each other, we just KNEW." Uh, or did they? A Swiss psychology grad student, Florian Zsok, ran some experiments to see what love at first sight is actually made of.

Zsok and his colleagues were looking for the three elements that psychologist Robert Sternberg theorizes interact to produce love: intimacy, commitment, and passion (made up of physical arousal, desire, excitement, and longing). They surveyed participants online and in a lab setting -- asking them how they felt about people in photographs -- and in three dating events, getting their reactions to people they'd just met. Of the 396 participants, love at first sight "was indicated 49 times by 32 different individuals." (That rare and wonderful lightning struck twice or maybe three times for some.) And here's a shocker: "None of the instances of (love at first sight) was reciprocal."

Not surprisingly, none of the participants who said they'd felt love at first sight had the elements of intimacy or commitment as part of their experience. The one element they did have? Passion -- in the form of "physical attraction." Basically, the researchers empirically confirmed what some of us intuitively understand: "Love at first sight" is just a classier way of expressing the sentiment yelled from passing cars: "Hey, miniskirt! You're late for your visit to My Penis Avenue!"

As for couples who insist they had love at first sight, the researchers believe they could be retrospectively repainting their first meeting to make their relationship feel more special. The reality: "We just knew" is "we just got lucky" (stated in a way that makes frustrated single people long to commit hara-kiri with the nearest shrimp fork).

Reminding yourself that you just have the plain old hots for this girl is probably the best way for you to do what needs to be done -- shift to some other activity (Masturbate! Play video games!) when the impulse strikes to stake out Coffeeland. Getting stuck on a total stranger this way probably makes it impossible to behave normally in their presence -- or want to look closely enough to see who they really are. As alluring a concept as love at first sight is, in practice it tends to work out best with inanimate objects -- a painting or an antique chair (something that doesn't make big wet smacking sounds when it chews or take so long to text you back that you buy it a burial plot).

My family enjoys your weekly column, but we're wondering why you can't give advice without launching into evolutionary explanations. We aren't always instinct-driven animals like elk or migrating salmon.

--Evolutionary Overkill

It isn't so bad being a salmon. Salmon just wake up one day and swim like mad upstream. There's no existential fretting, "What does it all mean? What will I do with myself after grad school? Am I a bad fish if I sometimes long to put grain alcohol in the sippy cup of that brat screaming on the beach?"

Meanwhile, back in humanland, research in cognitive neuroscience (by Michael Gazzaniga, among others) and in social science finds that we humans aren't the highly rational independent thinkers we like to believe we are.

In fact, as evolutionary psychologists Leda Cosmides and John Tooby put it, "our modern skulls house a stone age mind" -- adapted to solve hunter-gatherer mating and survival problems. This 10-million-year-old psychology, still driving us right now, today, is often a mismatch with our modern environment.

Take our sugar lust, for example. This made sense in an ancestral environment, where eating a couple of berries might have helped prevent malnutrition. Today, however, we can drive to Costco and have some guy load a pallet of doughnuts into our SUV while we burn .0003 of a calorie watching him.

Understanding the origins of our motivation is not "evolutionary overkill" but our best shot for possibly controlling our behavior -- or at least forgiving ourselves when we fail miserably.

As my First Amendment lawyer friend Ken White (@Popehat) tweeted about S'mores Girl Scout Cookies: "I thought they were kind of meh at first but by the third box I ate in the garage they were growing on me."

I'm a 35-year-old woman. I'm living with my boyfriend, who's a freelance artist (talented but just getting started). We've been together for three years, and I am paying for pretty much everything. I don't feel resentful. I feel like we're a team and eventually his career will take off. However, my parents keep saying it's a bad dynamic: I'm coddling him, and he's taking advantage of me.

--Worried

Ideally, when one partner is the sole breadwinner, the other is the stay-at-home parent to more than two rambunctious goldfish.

There's a term in risk researcher and former derivatives trader Nassim Nicholas Taleb's books -- "skin in the game." That's what's missing when, say, a hedge fund honcho advises you to make some big-bucks investment. If he's guessed right, he'll share in your profits. However, any losses are all yours -- as in, you'll find him up in his penthouse, not two cardboard boxes down from your new "home" on the corner.

"Skin in the game" is also what's missing from your boyfriend's end of the relationship. You're doing all of the work to keep the roof over the relationship. Your boyfriend's doing none of the work but reaping 100 percent of the benefits. This isn't to say relationships have to be exactly 50-50. But such a gross asymmetry in effort may be creating a breeding ground for laziness -- setting your boyfriend up to go all Leisure Larry in both his work and the relationship.

In fact, by making things so easy for him, you may be making it harder for him to succeed. Consider that you put in more effort when it's a necessity -- when you don't have inherited wealth (or a 9-to-5-toiling girlfriend who allows you to live as if you do).

Also, the fact that you're a woman who's paying for everything may make this more of a problem. Women evolved to seek "providers," and men coevolved to expect that -- and to expect the best women to hold out for investment. Men's self-worth is also driven by their ability to provide. So though many couples think they "should" be okay with a woman as the sole or primary moneymaker (because...equality!), it often leads to resentment in the woman and emasculation in the man. (Great if you like your sex without those boring erections.)

Finally, consider whether you really aren't okay with this Vincent van No Job arrangement but are going along with it because you think it's the good-girlfriend thing to do. It's okay -- and probably good for your relationship -- to ask your boyfriend to put "skin in the game," like by driving a bunch of runs on Uber to fork over for the electric bill. People value and feel more a part of something they have to work for -- and not just by opening all the bills (with an artistic flourish!) before handing them over for the wage slave girlfriend to pay.

I'm a 28-year-old gay guy. I like to travel and go out and do stuff on the weekends. My boyfriend prefers to smoke pot and uhh...time travel on the couch. He's a good person, and I love him, but he's unwilling to cut back on his pot smoking. Friends tell me to dump him, but we've been together for three years, and bailing now would mean throwing that time away.

--Frustrated

The guy isn't without ambition. He tries really hard every day to give the cat a contact high.

There's a point when love seems like "the answer" -- when you're 14 and practicing your make-out skills on your pillow. But then you grow up and get into a relationship with a man you love, and you find yourself packing for Bali while he's packing his bong.

Presumably, you've tried to come to some compromise. It helps to be specific about what would work for you -- like by proposing he come down from Weed Mountain to spend Saturday afternoon and evening out on the town with you. If he's unwilling to be enough of a boyfriend to make you happy, well, you have a decision to make.

In making it, don't let yourself get tripped up by "the sunk cost effect." This is decision researcher Hal Arkes' term for our (irrational) "tendency to continue an endeavor once an investment in money, effort, or time has been made." But that initial investment -- for example, the three years you've already put into your relationship with James Bong -- is gone. What makes sense is looking at whether the "endeavor" will pay off in the future, say, in a willingness by your boyfriend to combine his favorite hobby and yours. As travel writer Rick Steves put it, "I have used cannabis all over the world." (Hmm...then again, so have other people, and they're still in jail in Turkey.)