Skip to content

Transgender people: simple science

We don’t know for sure why some people are transgender. Some scientists are looking for the answers. This is what they know right now.


A human who works in science is called a scientist. Some of them study life. The study of life is biology. A person who studies biology is a biologist. Some biologists study reproduction. Reproduction is how living things make more living things.

The science of reproduction is going to change a lot by the end of the 21st century. That is why some people are scared of transgender people. They are often scared about what will happen when we can change humans and make new ones using new science.

How are human babies made?

Adult humans can make more humans by sexual reproduction. Adults can often make a gamete inside of their bodies. A gamete is a special kind of cell that is also called a haploid. There are two kinds of human gametes: eggs and sperm. Almost all humans are made from one egg and one sperm.

Most human babies have nine months of development from being made to being born. This development time is called gestation. At first, a sperm can join an egg. This is called fertilization. After fertilization, the joined egg and sperm haploids form a diploid cell called a zygote. When the zygote starts to grow one day later, it’s called an embryo. The embryo grows for 8 weeks. Then it’s called a fetus until it is born. After it’s born, it’s called a neonate (newborn) or an infant.

What is sex?

Sex is part of the development of a fetus. As it grows, a fetus develops into someone who can make eggs, sperm, or neither. This is called sex development. Five things cause sex development:

  • genes: these chemicals have a code that tells your body how to make the things your cells need. Inside each cell are structures called chromosomes. Each chromosome contains many genes. The code on every gene affects how you look (traits) and how you act (behaviors). Learn more at genetics.
  • sex hormones: these chemicals can tell your cells how to help make gametes. They can also make your body ready to give or receive someone else’s gamete. Sex hormone affect two kinds of sex characteristics:
    • Primary sex characteristics
      • gonads: the organs in your body that can make gametes.
      • internal genitalia: organs inside your body that can help give or receive someone else’s gamete.
      • external genitalia: outer organs that can help give or receive someone else’s gamete.
    • Secondary sex characteristics
      • breast tissue
      • body hair including underarm and pubic hair
      • fat percentage and distribution
      • skin including scent and sweat
      • skeleton

For a long time, people would often look at the external genitalia of an infant right after the infant is born. Based on what they saw, they would often say “this baby is male” or “this baby is female.” Saying a baby is male or female is called a sex assignment. Sometimes it is hard to tell just by looking at external genitalia. This is called a difference of sex development.

Some differences of sex development can not be seen with the eye. For instance, some people have genes that are unusual. They may have an extra chromosome that changes how their cells are made. They may have a gene that keeps sex hormones from working on them. They may have a gene that made them be born without some parts. There is also science that shows that genes can affect how the brain grows.

There is also science that shows that genes can be changed in some people by their environment. This is called epigenetics. Learn more at genetics.

What is gender?

Gender means type or kind. Some languages have a gender for some words. The gender can be:

  • masculine
  • feminine
  • neuter

The idea of gender is also used for how types of people look and act. Some cultures think of gender as having only two types: masculine and feminine. This is called a gender binary. Some cultures have a third gender, so gender does not always match up with sex.

Some scholars who study the idea of gender say that gender is a social construction. They argue that every culture makes up its own ideas about how gender is used on people. Some argue that sex is a social construction, too. They believe that organizing people and culture on the basis of sex will change a lot this century.

Some scholars who study the idea of gender think of it like a rainbow, where all the colors blend. Imagine someone saying “There are only two colors: warm and cool.” Someone else might say, “There are seven colors: red, orange, yellow, green, blue, indigo, violet.” Someone else might say “There are 360 colors, one for each unit of visible light.” People who think of gender like a rainbow call it a gender spectrum.

In a perfect world, everyone would be treated the same. Some people do not treat other people the same based on their sex assignment. They make up rules for how people should look and act based on their sex assignment. This is called a gender role. They also make up rules for how people should act toward others based on their gender. When these rules are not fair, it is called sexism.

For instance, these days many people test a fetus before it is born. One kind of test called an ultrasound can make an image of a fetus. That image can show the external genitalia. Before the infant is born, some people will choose how to treat the infant after it is born. They might buy clothes they consider “feminine” or toys they consider “masculine.” Those decisions are sexist.

What is gender identity?

When most children are around 3 years old, they start to form a gender identity. They start to think of themselves as part of a type based on gender roles in their culture. Many children form a gender identity that fits their sex assignment, but some do not.

What is gender expression?

Gender expression is how you look and act based on your culture’s gender roles. Most doctors who treat children say that letting all children wear or play with what they want is good for them. This is called an affirming model of care. They say it is better to support a child as part of healthy development.

When gender identity and sex assignment do not fit together

Some children form a gender identity that does not fit their sex assignment. This is called a difference of gender development. Most doctors who treat children say that these gender diverse children should not be punished or made to feel bad. Their gender identity may change over time if they have support from parents.

People who think gender is binary often think that gender identity is also binary. They often think gender diverse children are sick. They think the children can be cured by using shame and punishment. This is called a non-affirming model of care. They say that if this sickness is not treated, the child may become transgender. If it is treated, the child might not become transgender. People who think transgender people are sick call this cure a “desistance” or “detransition.” Many believe the affirming model is bad for children.

Ethics are a set of rules about what is good and bad. Many people who work with children and transgender people think the affirming model is good, or ethical. Many people who work with children and transgender people think the non-affirming model is bad, or unethical. In some places, the non-affirming model of care is against the law.

It is hard to talk about the science of gender identity and expression. Some people think our gender identity and expression can be a disorder or disease. Others think they are just how we feel and act (traits and behaviors). The two groups see the world in very different ways. They even use the same words in very different ways.

The people who think being transgender is a disease are not bad people. Even some transgender people think they have a disease. Good people can disagree.

Good science uses words and ideas that do not have bias. Bias means liking one idea more than others. Scientists should not have bias. Bias can cause bad science. Bad science can hurt people. That is why it is important to look for bias in all science. This is even more important when looking at science about people.

Some people claim transgender people hate science. This is a form of bias. Many famous transgender people are scientists. Many of us who are not scientists know a lot about how science works. We know that bad science has hurt transgender people in the past. We want all science to be good science.

Learn more

The next page with easy words is transgender politics.

This page uses easy words. This helps young people read it. This also helps people who don’t know many English words. The words in bold are hard. You need to know what they mean, or this will be hard to read. You can use these links to look up words you don’t know:

Merriam-Webster’s Learner’s Dictionary

Cambridge Learner’s Dictionary