"It was something in his eyes," she later said. "There was this remorse, sadness. I was attracted. I knew he was the one."

Within the year Polzin and Harris were engaged and she had moved to America to live with his family. This story seems a little surprising, but if you see the picture that Dagmar fell in love with it is, frankly, astonishing. He may have many charming accomplishments to recommend him as a husband, but Harris is not a bonny boy.

Polzin's romance is not an isolated incident: no matter how extreme or appalling the crime with which they are associated, it seems there is always a woman keen to stand by the man. It was recently reported that Ian Huntley, the Soham man charged with the murders of schoolgirls Jessica Chapman and Holly Wells, receives bundles of fan mail from women every week - many containing photographs of themselves.

Prison romances seem in no danger of dying out. But the cliche of the prison bride as wig-wearing trailer-trash is misguided: the women come from all sectors of society. Carlos the Jackal become engaged to his lawyer last year. The famous Glasgow hard man Jimmy Boyle married a psychiatrist he met in prison. The most common form of contact, certainly for many of the 100 or so British women currently engaged or married to American men on death row, is through anti-death-penalty campaign internet sites.

These correspondence schemes provide heart-wrenching photographs of young men alongside explanations for their crimes and pleas for contact. One young Alabama death row inmate ends his request for pen pals with the statement "loneliness is a terrible thing"; another finishes, "a friend is waiting". All promise to reply to any letters.

In her book, Women Who Love Men Who Kill, Sheila Isenberg examines the phenomenon of prison lovers and finds genuine and universal bewilderment among the women at their situation. Even if they have had a series of romances with prisoners or, like one British woman, been engaged to several death-row inmates - all of whom were executed - they still claim not to have chosen that course for themselves. Karen Richey's partner, for instance, is on death row in Ohio. Karen says that she wasn't looking for a love affair when she made contact with Kenny, a 38-year-old Scot: "My war cry is that I only wanted to be a pen pal. Kenny insists this is going to be on my grave stone."

It takes considerable effort to meet men in secure containment facilities. Many women will write to a number of prisoners before they finally make a sustainable connection. They may even take on voluntary jobs in prison, or go on blind-date visits with men they know only by reputation.

As on the outside, famous people attract a disproportionate amount of attention because of the glamour that surrounds them and ordinary people's desire for vicarious celebrity. Serial killer Richard Ramirez, the so-called Night Stalker, who murdered and dismembered 13 people in the 1980s, had no trouble finding a bride. Doreen Lioy started writing to Ramirez after falling for his picture in the paper. They were married in 1996 in the prison waiting room.

Both Ramirez and Ted Bundy, a rapist-murderer who was suspected of murdering 35 young women, attracted gangs of admiring groupies who sat patiently through their court cases. Even John Wayne Gacy - not the most eligible man, with a history of drugging, raping and murdering 30 young men in Chicago - ended up marrying a woman he met while awaiting the death penalty.

So what other reasons could there be for so many women being attracted to convicted criminals? Isenberg suggests that vicarious murder may sometimes be a motivating factor. It is easier for the lovers of these men to overlook violence if they have considered it themselves: "Even while she denies his culpability, it is his ability to murder that attracts her. He acted on his rage, however unsuitably. [The woman] could never act on her rage. So [his] murder is [her] murder," she says.

It is certainly true that many prison brides have a history of violent relationships. Isenberg draws positive conclusions from this, arguing that an imprisoned partner may be a healthy strategy for women who are attracted to violent men, allowing them to engage without putting themselves in physical danger.

Religious fervour is another, more obvious motivator. Evangelical Christian schemes bring women into contact with prisoners and provide a basis for intense emotional interaction.

Jacquelynne Willcox-Bailey's book Dream Lovers: Women Who Marry Men Behind Bars is a series of interviews conducted with Australian women. The most melancholy story concerns two middle-aged Christian sisters, Avril and Rose, who left long-term "boring" marriages for men in prison. One man had been convicted of a string of minor property offences, the other man had killed his previous wife. His new wife, Rose, said: "I have faith that if you're genuine with the Lord you're a new person. A lot of people have said I should be worried about him because of what he did and his background - which is pretty awful and violent - but I have no fear."

Despite the women's faith, both relationships ended tragically: a week after his release the thief bludgeoned Avril to death with a hammer. The other husband ended up back in prison after trying to cut Rose's ear off and pull out her teeth with pliers.

However, it is rare that the most disturbing type of relationship is formed. Hybristophiliacs are sexually excited by violent outrages performed on others. These women often send pornographic pictures of themselves to prisoners. The self-styled "most violent prisoner in Britain", Charles Bronson, publishes photos he receives on his website.

But not all hybristophiliacs are passive admirers. A playwright named Veronica Lynn Compton began a torrid affair with one of the Hillside Stranglers, a pair of cousins who abducted, raped and mutilated very young women before ritualistically displaying their corpses on hillsides in Los Angeles in the 1970s. As part of an elaborate defence strategy, one of the stranglers, Kenneth Bianchi, asked Compton to kill a woman using his modus operandi.

DNA evidence was not then available - only the blood type could be determined from a fluid sample - so he asked her to sprinkle the dead body with his sperm, and passed her a sample in a rubber glove. Compton tried but bungled the attempt and her prospective victim got away. By the time Compton was imprisoned for the attempted murder, Bianchi had married a different woman. Compton found another sexual serial killer to romance. One year he sent her a photo of a decapitated female corpse as a Valentine card.

But most prison romances are not so extreme. Generally the women are decent, well-meaning and it is easy to see why they find their relationships fulfilling. Their boyfriends spend their days exercising and their evenings writing letters and poems or trying to phone home. They are more compliant and attentive than they would be on the outside because the women send money, pay for their legal representation and afford them the tremendous parole advantage of a permanent address.

Prison relationships retain the intoxicating elements present in every romance. The first endorphin-flush of love always involves a degree of transference; we all see our partners as we hope them to be, imagining that the love object embodies the qualities we crave. Polzin projected remorse into Harris's puffy eyes. It is only as the initial infatuation ebbs that we begin to realise which of those assumptions were actually true.

Woman with imprisoned partners have limited contact and need never move beyond this courting stage. The intense desire for each other need never translate to the ordinariness of sex and marriage.

But, as clinical psychologist Dr Stuart Fischoff says, the love object is "almost irrelevant at this point. He's a dream lover, a phantom limb". Such fantasy projection can be used to wish away any aspect of reality. The excuses the women give for their partner's alleged crimes operate as in all other relationships. They do what we all sometimes do when faced with negative information about loved ones: they refuse to believe it.

On one website devoted to Richard Ramirez his wife says, "I appeal to all intelligent persons not to believe everything that is being presented about Richard in the media. The facts of his case ultimately will confirm that Richard is a wrongly-convicted man, and I believe fervently that his innocence will be proven to the world."

One lawyer, who uses her official visits to have sex in the interview room with a man convicted of a violent assault, sums up what many feel about prison romance: "There are lots of sad relationships in prison. A lot of opportunistic, shallow, revolting relationships and a lot of sad, hopeless people clinging to each other."

This is the most pronounced parallel with more conventional relationships: we can always see the truth about other people's relationships more clearly than our own.

· Denise Mina's latest crime novel, Sanctum, is published by Bantam Press, £16.99