Double the Fun: Fraternal Twin Facts

  • By: Sandra
  • Date: March 18, 2022
  • Time to read: 10 min.

Fraternal twins are the most common type of twin. They happen when two separate eggs are fertilized by two different sperm cells, and they share an egg sac. The egg sac splits in half early on, so each fetus has its own amniotic fluid and placenta.

Fraternal Twins Are Essentially Siblings Born on the Same Day

When compared to identical twins or monozygotic twins, who have 100 per cent of their DNA in common, fraternal twins have only roughly 50 per cent of their DNA in common. As a result, they are no more genetically identical to one another than non-twin siblings.

For years, researchers have studied the six twin subgroups—identical twin boys (MZm), identical twin girls (MZf), fraternal twin girls (DZSSf), fraternal twin boys (DZSSm), and of course, opposite-sex twins, DZOSm and DZOSf—and have come up with a list of general characteristics for each.

Fraternal Twins are the Most Common Type of Twin

Fraternal twins account for around 75% of all twins and can be boy/boy, girl/girl or girl/boy twins. The gender of a child is determined by the chromosomes in the father’s sperm, just as it is with any other conception. Because fraternal twins are created by the fertilization of two distinct sperm, each of the two eggs can be either a boy or a girl. As a result, fraternal twins can be both boys or both girls, or they can be both boys and girls. 

Fraternal Twins Can Have Different Fathers!

Whilst highly unusual this can happen in a process known as superfecundation or bi-paternal twins. This occurs when a woman releases many eggs (a condition known as hyper-ovulation), has sexual relations with more than one partner, and the eggs are fertilized by sperm from each of the partners involved in the relationships.

Fraternal Twins Can be Totally Different

Due to the fact that fraternal twins only share 50% of their DNA, they can have vastly distinct personalities and interests. As a result, while you will surely appreciate their individuality, you may find it difficult to master their sleep regimen. As a result, identical twins are typically more closely related than fraternal twins. The relationship between identical twins and their parents is stronger than that between fraternal siblings since they share 100 percent of their DNA.

However, conversely, they can also look like identical twins and be extremely similar looking. The only way to really tell whether twins are fraternal or identical is to do a DNA  zygosity genetic test.

Fraternal Twin Girls: 5 Things Parents Need to Know

Fraternal twin boys are boisterous and assertive. As toddlers, they’re often aggressive. And without a doubt, young fraternal twin boys are exhausting! These characteristics are also based on research but in no way are set in stone—you may find that your girls fit them to a tee while others of you won’t be able to relate. At. All. Nonetheless, it’s fun to see what science has to say about our twins. So here we go—Fraternal Twin Girls: 5 Things Parents Need to Know.

Fraternal twin girls excel in speech, even—at times—surpassing their singleton counterparts.

We all know that at an early age, girls excel verbally when compared to boys. Studies have shown that they tell longer, more imaginative stories than male twins. But did you know that fraternal twin girls often have better articulation than even singleton females of the same age?

Fraternal twin girls are very sociable.

Yes, these girls are true social butterflies. But what’s even more fascinating, however, is that when they share a classroom, their peers perceive them as more popular than their singleton, female classmates. Furthermore, fraternal twin girls seem to be aware of their twin status and use it to their social advantage, culling together a large group of friends. In other words, this is an affable group of girls. They know how to work it!

Although many are close, fraternal twin girls yearn to be seen as individuals more than the other twin subgroup.

Research has shown that when this group reaches the school years, they often prefer to play alone than with their cotwin. Not only do they tend to build separate friendships more often than any other twin subgroup except boy-girl twins but they also feel less accepted by their cotwins’ friends. Yet, it’s important to add that these same girls also report being emotionally available to their cotwins.

So how is this knowledge helpful?

Fraternal twin girls seem to be the most independent. Don’t fight it; encourage their self-sufficient spirit! This doesn’t mean they don’t like each other. To the contrary, many fraternal twin girls are very close. They just don’t have the need to be with each other 24/7. As a parent, never insist that one twin take the other with her on a play date, or worse, to a birthday party where only one was invited.

When compared to other twin subgroups, fraternal twin girls describe themselves as the most different from each other.

From their private thoughts and preferred choice of activities to their own personal style of dress—hair included—these girls think they are the most different from each other when compared to all other subgroups of twins (identical girls, identical boys, fraternal boys, and boy-girl twins). They simply don’t believe it’s important to be alike. Furthermore, they don’t always like to share their possessions with their cotwins. Yet these girls don’t look at these traits as negative; it’s simply a self-observation of their relationship as a whole. It’s just who they are.

How is this knowledge helpful?

Respect their privacy. Don’t insist that they share everything. If you can offer each her own room, go for it. If you don’t have the space, find subtle but effective ways to give each a sense of solitude. When parents respect the wishes of each individual twin, the inter-twin relationship is free of constraints and will blossom.

Puberty can be a tumultuous time for fraternal twin girls.

Although identical twin girls often reach menses within days of each other (although it’s not uncommon for the span to reach six months), fraternal twin girls are different. With only approximately 50 percent of their DNA shared, these girls, as you know, are merely siblings born on the same day. Therefore, they can hit puberty years apart! Translation: Hang on for a bumpy ride!

How is this knowledge helpful?

Spending lots of individual time with each girl during early puberty can go a long way to counteracting any animosity that one twin may feel for the other. (“Why does she get to wear a bra first?” or “How come the boys like her so much?”) Allow each daughter some alone time where she can vent her frustration. Acknowledge her feelings and offer loving reassurance.

As teenage girls are particularly aware of their personal appearances, don’t compare their looks to one another, and certainly intervene when a relative or friend feels the need. (It seems like no-brainer, I know, but it’s worth repeating!) Furthermore, expect conflict. Fighting! Bickering! All kids fight, and twins are no exception. Dealing with the drama is all part of parenting teens. But set reasonable limits.

Fraternal Twin Boys: Five Things Parents Need to Know

The following list of five things you should know about your fraternal twin boys is here to act as a guide, and perhaps offer you just a little peace of mind. But remember, these are just generalities based on research from a control group of twins. Your twins may be the exception to the rule and defy them all. And while it’s never a good idea to stereotype any group, it’s helpful to understand the subtle variations that from time to time crop up among all the different types of twins. 

As toddlers, fraternal twin boys will wear you down to the point of exhaustion. 

Unlike twin girls who tend to sit and play together for lengthy periods of time, fraternal twin boys are often more physically active, preferring to examine their worlds through action. I know this first hand as I’ve often written about those chaotic toddler years. I can remember heading to the store with my toddler boys and from the moment we ventured inside, they’d take off running in opposite directions. Not because they wanted to be naughty and deliberately break things. No, they were good kids. But they were curious little guys and wanted to see and touch everything.

So how is this knowledge helpful?

Just knowing that the craziness is normal can be reassuring (to a point, of course). From the moment your toddler twin boys wake in the morning to the moment their heads hit the pillow at night, they will be on the go. That’s just the way it is. In order for you to survive, however, you’ll need to prepare. That means keeping those twins busy! From weekly play dates and park outings to high-quality preschool that focuses on active play time, if your boys are engaged, they’ll be happy. And when they’re happy, so are you! Remember, when you have toddler twin boys, there’s no such thing as “over scheduling.”

Research shows that fraternal twin boys tend to be the most aggressive of all twin subgroups.

That explains the constant hitting, biting, pushing and arguing, doesn’t it? But on the plus side, fraternal twin boys also exhibit high leadership skills and are more assertive in articulating or expressing what they think are their rights. (“Hey, that’s my toy!”) It may drive you crazy now when they’re toddlers as it’s not easy arbitrating their “toy wars,” but it will serve them well once they hit the school years.

They may not be the best of friends…for now.

Although many fraternal twins are close, calling each other “best friend,” many fraternal twin boys prefer the company of someone other than their cotwin, especially when they hit the school years and begin to bond with their classmates. This may never be an issue in your house if both boys are on the same page, each having an outside best friend. Or, it can be the source of pain and conflict if one twin wants to continually find outside play dates at the exclusion of his cotwin.

So how is this knowledge helpful?

It’s all normal behavior. Try to remember that they are merely two siblings born on the same day and may have two very different temperaments, personalities and passions. Don’t try to shame or guilt the independent twin into hanging out with his brother when he may not want to (“But he’s your twin!“) as it will surely backfire. Instead, respect his right to have outside friendships and then encourage the dependent twin to do the same. When you allow each to be an individual, something magical happens—they are then free to be friends with each other again, no strings attached.

Fraternal twin boys are usually more competitive and rivalrous than other twin subgroups.

From the time they were little, my fraternal twin boys have always tried to outdo each other. It used to be a competition to see who was taller; now it centers mostly on who gets better grades in school. (“Ha-ha! I got two point higher on my math test!” or “I scored 100 points higher on my SATs!”) It never seems to matter if they don’t quite measure up to their friends; they just want to eclipse their cotwin.

So how is this knowledge helpful?

Ignore it. Don’t play into it. A little bit of twin rivalry is actually healthy as it teaches kids how to negotiate and handle conflicts. Plus, I think they truly enjoy the verbal sparring with one another as it gives each the opportunity to fine-tune his tongue. (I wonder if a large number of lawyers are fraternal male twins!)

But if you sense that your twins’ competition is getting out of control, take a look at your own behavior. Be very careful not to openly compare your twins even with a seemingly innocent positive comparison (“Wow! You did great on your test! You scored even higher than your brother!”) You may want to consider placing them in separate classrooms at school, too, or offer them the chance to play on different sports teams. Finally, make an effort to spend alone time with each twin individually.

Fraternal twin boys may be very physically different.

Different genes, different people. It’s bound to happen. One twin may be blonde, the other a red-head. One boy may need glasses, the other may have 20/20 vision. It’s all par for the course and for the most part readily accepted by the twins themselves.

So how is this knowledge helpful?

But then there’s the issue of height. To many fraternal twin boys, being shorter than your classmates is bad enough but to have your cotwin tower over you by several inches hurts all the more. If you feel that the physical differences between your twins is a sensitive issue, loving reassurance can go a long way. For instance, did you know that thin siblings usually reach puberty at a later age than even slightly overweight siblings? Furthermore, boys are lucky in that they can continue to grow well into their late teens and early twenties (compared to girls who usually stop growing shortly after their first periods.)

And when friends and family publicly point out your twin’s differences (“Isn’t it funny how Bobby is a full head taller than Jeffrey?”), step in discreetly asking them to table the conversation.