As a woman who has chosen not to have kids, Amy Blackstone always wanted to understand all the different reasons people opted out of parenthood.

“I really had this feeling that something must be wrong with me,” Blackstone, a sociology professor at University of Maine, told TODAY.

“Why is it that I’m not feeling this pull toward motherhood that we’re all told, as we’re raised, all of us will feel?”

She set out to answer that question by interviewing 21 women and 10 men who also chose to be childfree. It’s a small sample, but the study offers a good glimpse into people’s decision-making, Blackstone noted.

And yes, “childfree” is the preferred term, she said. She uses “childless” to refer to people who would like to be parents, but can’t. Some, including actress Kim Cattrall, find the description offensive.

Actress Kim Cattrall dislikes the term "childless." Frederick M. Brown / Getty Images

“It sounds like you’re less, because you haven’t had a child,” the “Sex and the City” star told the BBC last year.

RELATED: Child-free by choice: How Cameron Diaz represents women like me

Adults who are opting out of parenthood are a growing population, Blackstone’s study notes. Almost twice as many U.S. women ages 40 to 44 did not have kids in the 2000s than in the 1970s.

About 15 percent of U.S. women and 24 percent of men have no children by the time they reach 40, according to the National Survey of Family Growth.

Here are the common themes Blackstone found among people who were able to have kids, but decided not to:

1. Remaining childfree is a conscious decision, not an accident.

There is a common perception that people who decide not to have children must have intended to get pregnant at some point, but just never got around to it.

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That’s not the case. The majority of participants said staying childfree was a conscious decision.

“People who have decided not to have kids arguably have been more thoughtful than those who decided to have kids. It’s deliberate,” a male study participant said.

“Most people who have children don’t even think about it, they just have them,” a woman added.

RELATED: Are more couples going childless by choice?

2. I came to the decision over time, not because of a single event.

People in the study described it as a “working decision,” happening over the course of a person’s life and influenced by childhood experiences, personal attitudes, conversations with partners and observing people with children.

3. ‘I always kind of felt this way.’

On the other hand, a couple of the participants always knew having children wasn’t something they wanted.

4. I didn't like how other people’s lives changed when they had kids.

Many of the study participants carefully observed others in their circle who became parents and didn’t like what they saw.

“As my friends started to have kids, that made me go, ‘Oh, I don’t think this is for me.’ Because even if I had wanted kids before that, once they started having kids and losing their freedom and their individuality, that really was a big point for me. It was like, that does not look like the fun, happy family stuff that you think about when you’re young,” one woman said.

“A lot of people with children didn’t look happy... The majority were definitely stressed out. There was something there that was not inviting me to participate in this lifestyle process,” another noted.

She has a point. U.S. parents generally are not as happy as people who don’t have kids, a recent study found.

RELATED: Childlessness is up in the U.S.

5. I want to be close to my partner.

For some people, the motivation not to have kids was very similar to the motivation many parents cite for having them: a longing to have a strong relationship in their lives.

Many of the study participants carefully observed others in their circle who became parents and didn’t like what they saw. Getty Images

In the case of parents, it’s an interest in nurturing a child; for childfree people, it’s all about intimacy and focusing their love on their partner, Blackstone said.

Previous studies have found couples who don’t have kids have happier marriages.

6. Having children would put limits on what I want to do in life.

This was a more common theme among men. They really considered how parenting would impact their lives and what they would have to give up if they had children. An impromptu trip to Paris, skydiving lessons or a cute convertible for two might be out if you have a baby at home.

The majority of the study participants said staying childfree was a conscious decision. Shutterstock

In general, men tended to consider themselves while making the decision and their process was more internal, personal and individual. Women, on the other hand, thought about others and framed it as a decision made together with their partners.

7. It’s the responsible decision

Women in particular thought about how having kids would impact the environment, over-consumption and over-population; and whether it would be fair to bring a child into this world.

“I camped over the weekend and I saw the trash factor that people with kids had left and let build up from so much over use of a camp site. I think about stuff like acceptable population levels,” one woman said.

“I really think that the world is against the child right now. At this time in our social structure right now it’s not going to be a good thing to have children. We can’t bring them up healthfully,” another added.

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