For Carren Strock, the revelation came when she was 44. She had met her husband – "a terrific guy, very sweet" – at high school when she was 16, had been married to him for 25 years, had two dearly loved children, and what she describes as a "white-picket-fence existence" in New York. Then, one day, sitting opposite her best friend, she realised: "Oh my God. I'm in love with this woman." The notion that she might be a lesbian had never occurred to her before. "If you'd asked me the previous year," she says, "I would have replied: 'I know exactly who and what I am – I am not a lesbian, nor could I ever be one.'"

From that moment Strock's understanding of her sexuality changed completely. She felt compelled to tell her friend, but her attraction wasn't reciprocated; at first she wasn't sure whether she had feelings for women in general, or just this one in particular. But she gradually came to realise, and accept, that she was a lesbian. She also started to realise that her experience wasn't unusual.

Strock decided to interview other married women who had fallen in love with women, "putting up fliers in theatres and bookstores. Women started contacting me from across the country – everyone knew someone who knew someone in this situation." The interviews became a book, Married Women Who Love Women, and when it came to writing the second edition, Strock turned to the internet for interviewees. "Within days," she says, "more women had contacted me than I could ever actually speak to."

Late-blooming lesbians – women who discover or declare same-sex feelings in their 30s and beyond – have attracted increasing attention over the last few years, partly due to the clutch of glamorous, high-profile women who have come out after heterosexual relationships. Cynthia Nixon, for instance, who plays Miranda in Sex and the City, was in a heterosexual relationship for 15 years, and had two children, before falling for her current partner, Christine Marinoni, in 2004. Last year, it was reported that the British singer Alison Goldfrapp, who is in her mid-40s, had started a relationship with film editor Lisa Gunning. The actor Portia de Rossi was married to a man before coming out and falling in love with the comedian and talkshow host, Ellen DeGeneres, whom she married in 2008. And then there's the British retail adviser and television star, Mary Portas, who was married to a man for 13 years, and had two children, before getting together with Melanie Rickey, the fashion-editor-at-large of Grazia magazine. At their civil partnership earlier this year the pair beamed for the cameras in beautiful, custom-made Antonio Berardi dresses.

The subject has now begun attracting academic attention. Next month at the American Psychological Association's annual convention in San Diego, a session entitled Sexual Fluidity and Late-Blooming Lesbians is due to showcase a range of research, including a study by Christan Moran, who decided to look at the lives of women who had experienced a same-sex attraction when they were over 30 and married to a man. Moran is a researcher at Southern Connecticut University, and her study was prompted in part by an anguished comment she found on an online message board for married lesbians, written by someone who styled herself "Crazy".

"I don't understand why I can't do the right thing," she wrote. "I don't understand why I can't make myself stop thinking about this other woman." Moran wanted to survey a range of women in this situation, "to help Crazy, and others like her, see that they are not abnormal, or wrong to find themselves attracted to other women later in life".

She also wanted to explore the notion, she writes, that "a heterosexual woman might make a full transition to a singular lesbian identity . . . In other words, they might actually change their sexual orientation." As Moran notes in her study, this possibility is often ignored; when a person comes out in later life, the accepted wisdom tends to be that they must always have been gay or bisexual, but just hid or repressed their feelings. Increasingly researchers are questioning this, and investigating whether sexuality is more fluid and shifting than is often suspected.

Sarah Spelling, a former teacher, says she can well understand how "you can slide or slip or move into another identity". After growing up in a family of seven children in Birmingham, Spelling met her first serious partner, a man, when she was at university. They were together for 12 years, in which time they were "fully on, sexually," she says, although she adds that she has never had an orgasm with a man through penetrative sex.

Spelling is a keen feminist and sportsperson, and met lesbian friends through both of these interests. "I didn't associate myself with their [sexuality] – I didn't see myself as a lesbian, but very clearly as a heterosexual in a longstanding relationship." When a friend on her hockey team made it clear she fancied her, "and thought I would fancy her too, I was like 'No! That's not me!' That just wasn't on my compass." Then, aged 34, having split up with her long-term partner, and in another relationship with a man, she found herself falling in love with her housemate – a woman. After "lots of talking together, over a year or so," they formed a relationship. "It was a meeting of minds," says Spelling, "a meeting of interests. She's a keen walker. So am I. She runs. So do I. We had lots in common, and eventually I realised I didn't have that with men." While having sex with a man had never felt uncomfortable or wrong, it wasn't as pleasurable as having sex with a woman, she says. From the start of the relationship, she felt completely at ease, although she didn't immediately define herself as a lesbian. "I didn't define myself as heterosexual either – I quite clearly wasn't that. And I wouldn't define myself as bisexual." After a while she fully embraced a lesbian identity. "We've been together for 23 years," she says, "so it's pretty clear that that was a defining change."

Dr Lisa Diamond, associate professor of psychology and gender studies at the University of Utah, has been following a group of 79 women for 15 years, tracking the shifts in their sexual identity. The women she chose at the start of the study had all experienced some same-sex attraction – although in some cases only fleetingly – and every two years or so she has recorded how they describe themselves: straight, lesbian, bisexual, or another category of their own choosing. In every two-year wave, 20-30% of the sample have changed their identity label, and over the course of the study, about 70% have changed how they described themselves at their initial interview. What's interesting, says Diamond, is that transitions in sexual identity aren't "confined to adolescence. People appear equally likely to undergo these sorts of transitions in middle adulthood and late adulthood." And while, in some cases, women arrive at a lesbian identity they've been repressing, "that doesn't account for all of the variables . . . In my study, what I often found was that women who may have always thought that other women were beautiful and attractive would, at some point later in life, actually fall in love with a woman, and that experience vaulted those attractions from something minor to something hugely significant. It wasn't that they'd been repressing their true selves before; it was that without the context of an actual relationship, the little glimmers of occasional fantasies or feelings just weren't that significant."

Diamond has a hunch that the possibility of moving across sexual boundaries increases as people age. "What we know about adult development," she says, "suggests that people become more expansive in a number of ways as they get older . . . I think a lot of women, late in life, when they're no longer worried about raising the kids, and when they're looking back on their marriage and how satisfying it is, find an opportunity to take a second look at what they want and feel like." This doesn't mean that women are choosing whether to be gay or straight, she clarifies. (Diamond's work has sometimes been distorted by rightwing factions in the US, who have suggested it shows homosexuality is optional.) "Every one of the women I studied who underwent a transition experienced it as being out of her control. It was not a conscious choice . . . I think the culture tends to lump together change and choice, as if they're the same phenomenon, but they're not. Puberty involves a heck of a lot of change, but you don't choose it. There are life-course transitions that are beyond our control."

This was certainly true for Laura Manning, a lawyer from London, who is now in her late 40s. She had always had a vague inkling she might have feelings for women, but met a man at university, "a really gentle man, Jeff, and I fell in love with him, and for a long time that was enough to balance my feelings". She married him in her late 20s, had two children in her early 30s, "and once I'd got that maternal part of my life out of the way, I suddenly started thinking about me again. I started to feel more and more uncomfortable about the image that I was presenting, because I felt like it wasn't true." In her late 30s, she began going out clubbing, "coming back on the bus at four in the morning, and then getting up and going to work. I was still living with Jeff, and I just started shutting down our relationship. He knew I was pushing him away."

The marriage ended, and Manning moved out. She has since had two long-term relationships with women, and says she's much happier since she came out, but suspects that her biological urge to have children, and her genuine feelings for Jeff, made her marriage inevitable on some level. "The thought of sex with a man repels me now, but at the time, when I was in my marriage, I didn't feel that, and I didn't feel I was repressing anything. The intensity of feeling in my relationship with Jeff overcame and blanketed my desires for women."

Sexual fluidity occurs in both men and women, but it has been suggested that women are potentially more open and malleable in this regard. Richard Lippa, professor of psychology at California State University, Fullerton, has carried out a variety of studies that have led him to the conclusion that, "while most men tend to have what I call a preferred sex and a non-preferred sex . . . with women there are more shades of grey, and so I tend to talk about them having a more preferred sex, and a less preferred sex. I have definitely heard some women say, 'It was the person I fell in love with, it wasn't the person's gender,' and I think that that is much more of a female experience than a male experience.

"I've never had a straight man say to me, at age 45, I just met this really neat guy and I fell in love with him and I don't like men in general, but God, this guy's so great that I'm going to be in a relationship with him for the next 15 years." In Diamond's study, around a quarter of the women have reported that gender is largely irrelevant in their choice of sexual partners. "Deep down," said one woman, "it's just a matter of who I meet and fall in love with, and it's not their body, it's something behind the eyes."

When Tina Humphrys, 70, first fell in love with a woman, she didn't define herself as a lesbian, "I just thought: 'It's her.'" Humphrys was in her mid-30s, had two children, and was coming out of a horrible second marriage. "I hated my life," she says. "The four bedrooms, the children – well, I didn't hate them, they just bored me to tears. I used to lie on the couch and my eyes would fill with tears as they had their naps."

She had found women attractive in the past, "but I think women do, don't they? You look and you think – that dress looks fabulous, or isn't she looking slim, or doesn't she look pretty. But you don't necessarily put sexual feelings on it." Then she went to university as a mature student, joined a women's group, and started to fall for one of the other members. "It was a bit of a shock to find that I was attracted sexually to this woman, but then it was also a decision to leave men. It was a decision to leave a particularly oppressive and restrictive way of living and try to live differently." She moved into a "commune-type place", and had non-monogamous relationships with women for a while, before settling down with her current partner of more than 30 years. While she had had "a very active sex life with men", she enjoyed sex with women much more. "I was once doing a workshop with a woman who used to tear hideous things that had been said about women out of the paper, and she had a piece about this blonde model who had romped with a lesbian – because they always romp, don't they? – and she said: 'It wasn't proper sex, it was just a load of orgasms.'" Humphrys laughs uproariously. "I think that just about sums it up, doesn't it?"

Beyond the sex, Humphrys found a connection that was more intense "on every level" than any she had found with a man. Strock echoes this view. "I've run workshops with straight women, and I've asked them, did you ever feel those sky rockets go off, or hear the music playing, when you fell in love with that significant other? And very few raise their hands. And then I went to a gay women's group, and I said, how many of you have ever felt the same? And almost all the hands went up. So connections with women are very different to connections between women and men."

The psychotherapist and writer, Susie Orbach, spent more than 30 years with the writer Joseph Schwartz, and had two children with him, before the partnership ended, and she subsequently formed a happy, ongoing relationship with the novelist Jeanette Winterson. Orbach says that the initial love connection between mother and daughter makes lesbian feelings in later life unsurprising. "If you think about it," she says, "whose arms are you first in, whose smells do you first absorb, where's that body-to-body imprint? I mean, we're still not really father-raised, are we, so it's a very big journey for women to get to heterosexuality . . . What happens is that you layer heterosexuality on top of that bond. You don't suddenly switch away from it. You don't give up that very intimate attachment to a woman."

Of course, the notion that your sexuality might shift entirely isn't welcomed by everyone; as Diamond says, "Even though there's more cultural acceptance than there was 20 years ago, same-sex sexuality is still very stigmatised, and the notion that you might not know everything there is to know about something that's so personal and intimate can terrify individuals. It's really hard for people to accept." That's why the writing and research in this area is so important. When the first edition of Strock's book was published, "a woman came up to me at one of my early speaking engagements, clutching the book and sobbing," she says. "She thought she was the only married woman ever to have fallen in love with another woman, and had no one to talk to, didn't know where to turn. And she had decided that the best thing was to kill herself on a night when she knew her husband and children were going to be out late. She'd planned her suicide. She was coming home from work for what she thought would be the last time, and she passed a bookstore, and they were putting my book in the window, and when she realised that she wasn't the only one, she chose to live".

The late-blooming lesbians I spoke to had all found happiness on their different paths. Strock is still a lesbian – and also still married to her husband, who knows about her sexuality. "He would never throw me away, and I would never throw him away," she says, "so we've re-defined our relationship. I'm a lesbian, but we share a house, we have separate rooms, we have two grandchildren now, and our situation is not unique." Most of the other women I spoke to were in happy, long-term relationships with women, and had found a contentment that they'd never experienced in their previous relationships.

"While some people find change threatening," Diamond says, "others find it exciting and liberating, and I definitely think that for women in middle adulthood and late life, they might be the most likely to find sexual shifts empowering. We're an anti-ageing society. We like people to be young, nubile and attractive. And I think the notion that your sexuality can undergo these really exciting, expansive possibilities at a stage when most people assume that women are no longer sexually interesting and are just shutting down, is potentially a really liberating notion for women. Your sexual future might actually be pretty dynamic and exciting – and whatever went on in your past might not be the best predictor at all of what your future has in store."