March 15, 2012 -- Sarah, now 50, came out as transgender but was still "being clocked" as male.

"You are walking through the mall and someone turns and says, 'Oh, my god, it's a man dressed as a woman,'" she said. "They figure it out."

But that rarely happens anymore. Thanks to facial feminization surgery in 2007, Sarah, not her real name, is living with the face she was believes she was supposed to be born with.

It was the first big step in her new life as a woman and in 2009, Sarah went on to have sex reassignment surgery.

Now, the former electrical contractor said, "I get treated like any other woman there," she said.

What makes a person's face look feminine or masculine?

Humans worldwide have a "gut" reaction to whether a person is a man or a woman, and it is not based on the "obvious," according to Dr. Jeffrey Spiegel, chief of the Division of Facial Plastic and Reconstructive Surgery at Boston University Medical Center.

A woman's eyes and mouth reflect more light than man's, according to Spiegel, who did Sarah's surgery. Women's eyebrows are arched and reach a peak at the edge, and men's are slightly lower and straight across.

Women's lips are positioned higher on the face and more teeth are visible. Their chins also tend to be smaller and more tapered.

"It's hard to look female," said Spiegel, who calls himself "the best" in facial feminization surgery.

When research subjects are shown photos, even for just a fraction of a second, they almost always get the gender right. "When they make a mistake, it's always thinking that a woman is a man and never that a man is a woman," he said.

Spiegel views his work as "gender-confirming surgery," and the goal is to provide all the subtle facial features that support a person's sense of who they truly are.

"I didn't know what gay or transgender was growing up," said Sarah, who attempted suicide and was misdiagnosed as bipolar during her unhappy marriage.

"It was a major taboo in my generation," she said. "But I knew something was wrong."

Growing awareness among transgender people that medical help is available, as well as more information online has fueled Spiegel's business.

Since 2004, he has performed more than 500 feminization surgeries for patients from all over the United States and the world. Surgically speaking, it's more difficult to make a man's face appear feminine than the other way around, according to Spiegel.

His work incorporates several different procedures, such as jaw-narrowing techniques from Asia. He also considers other features, such as lip and scalp height and shadowing on the eyes.

All that scientists have attributed to feminine beauty is "outdated and the tenets are not accurate," said Spiegel. "Plastic surgery has completely changed what we think about beauty."

Does a beard or a bald head always make a person look masculine?

"If you see a woman who has a shaved head like Sinead O'Connor or a woman undergoing chemotherapy, you don't look and say, 'There's a man,' but 'There's a bald woman,'" he said.

"Similarly, if you see a bearded woman at the circus or a hirsute woman whose genes give her hair on the face, you think, 'That woman needs to do something about that.' But no one thinks it's a man dressed as a woman."

Women Look More Like Men as They Age

Biology plays are role in why men's and women's faces differ.

"It's true, women can be mistaken for men and there is an evolutionary explanation for why we look different," he said. "We are hard-wired to find a good fertile mate."

Eyebrows are the most important feature in determining what looks feminine or masculine and that's the reason why they stand out on an otherwise hairless face.

Men often have a classic, strong wide "Dudley Do Right" or Arnold Schwarzenegger jaw, according to Spiegel. Women's are more tapered.

The hairline is also a give-away. Men have a "widow's peak" that recedes on the sides. Women have a rounder hairline.

Women become more masculine-looking as they get older, and Spiegel is also seeing an increase in interest in surgery among women who are not transgender.

"Why are some women more attractive than others?" he asked. "A big part is as women age and lose their fertility, they lose their femininity."

Their eyebrows descend and get flatter, their cheeks are less full, and with the jowls of aging, the tapers of the jaw disappear.

Facial feminization surgery, which is elective, is expensive, ranging from a couple of thousand dollars for a simple procedure to $25,000 to $35,000 for a whole range of changes. And because it is not covered by insurance, many patients do the surgery in stages.

Spiegel gets letters and accolades from patients who say they feel they have been "reborn."

Chloe, a 28-year-old musician from New York, said Spiegel's surgical hand had transformed her life as a transsexual woman whose transition from male to female had been rocky.

"Oh my god, it completely changed everything," she said at the airport en route to Japan for a DJ gig. "No one even questions my gender."

Spiegel reshaped her nose, eyebrows and chin to make her look natural, even without make-up. "It's amazing what a few millimeters can do," said Chloe, not her real name.

She said the "external stuff" has not been as important as accepting herself.

"I think the ultimate goal of any surgery, regardless of whether you are transgender or not, is to do something for yourself -- to come from a place where you try to be happy for who you are and not try to please other people. If you chase for other's approval, you will fail."

Spiegel agrees and has found a patient's entire personality and outlook on life can improve with a few surgical tweaks.

One transgender woman who had struggled with alcohol addiction and depression during her marriage sought facial feminization surgery, but had no one to be with her except her estranged wife.

The patient was so happy with the results that she reunited with her ex-wife, who later told Spiegel, "Now I am with the person I love. The miserable, depressive alcoholic stage was over. I figured out what mattered."

"I like that I get to help people," he said. "And it's gratifying to be able to solve a problem that is so fundamental."

As for Sarah, she had "the whole menu" of surgical items: her eyebrows, hairline, cheeks and jaw were readjusted to make her look more like a woman.

She now works a "dream job" in the hospitality industry. And a new feminine face also had another benefit -- she looks younger.

"I am 50 and people think I am 35," said Sarah. "I never wanted to be with a man because of my body, but now, I can express myself."