👋 Happy Sunday! Axios editor (and fraternal twin) Carly Mallenbaum here with a special newsletter edition about twins, for Twin Cities, because she couldn't help herself.

• 🔍 Find these stories on our Wellness Brief page.

⚾️ Fun fact: MLB's "Minnesota Twins" were very nearly named the "Twin Cities Twins," Kyle tells me.

🎂 Happy early birthday to our member Marcia McCoy!

Today’s newsletter is 931 words — a 3.5-minute read.

In the last decade, the rate of twin births has declined overall in Minnesota and the country, per CDC data.

The big picture: Early use of IVF contributed to a national twin baby boom. But once fertility treatments got more advanced, doctors increasingly recommended transferring a single embryo to reduce pregnancy risks.

By the numbers: Minnesota's rate has largely remained higher than the U.S. rate, spiking to 34.5 twin births per 1,000 births in 2022, then declining to 31.7 by 2024, per the CDC.

Between the lines: The reason behind the state's 2022 twin birth increase isn't entirely clear, says Sylia Wilson, associate professor at the Minnesota Center for Twin and Family Research.

• Wilson initially wondered whether pandemic-era stress could have played a role, but research she later reviewed did not support that theory.
• Another possible factor: Minnesota saw relatively high birth rates among women in their 30s and early 40s in 2022 — age groups that are more likely to have twins.
• Those birth rates declined in 2023, the same year Minnesota's twin rate fell from its 2022 peak.

Twins are one of science's best tools for untangling nature from nurture, according to the Minnesota Center for Twin and Family Research.

Why it matters: Comparing identical twins (who have essentially the same genes) and fraternal twins (who share about half of their genes, like any other siblings) helps researchers estimate how strongly genetics and environment influence traits, behaviors and health.

Between the lines: If identical twins are about as similar to each other as fraternal twins are to each other on some trait, that suggests the environment is doing a lot of the work.

• If identical twins are more similar to each other than fraternal twins, that points to a stronger genetic influence.

What they're saying: Twin research has found that "almost everything is influenced by genes to some extent," says Sylia Wilson, associate professor at the University of Minnesota.

• That includes "personality, psychiatric conditions, likes, dislikes," she says.
• Decades ago, there was much more emphasis on family environment. That often meant "the default was to blame the family, and in particular the mother" when people were struggling, Wilson says.

Yes, but: Because twins start out with so much in common, researchers can also use their differences to see how individual experiences may affect the body.

What they found: In a twin study, Wilson and colleagues found that when twins differed in their alcohol use, the heavier-drinking twin tended to have a smaller hippocampus, a brain region involved with memory.

• "So that does suggest that there's some effect of alcohol on that particular region of the brain," she says.

Zoom in: In 2020, Wilson started an infant twin registry in Minnesota, continuing the work of her predecessors who kept records of identical and fraternal twins in the area.

• The registry invites families to participate in studies on behavior and relationships.

The bottom line: Twin research isn't just about twins. It's a tool for understanding everyone.

If you're not a twin yourself, you may not be familiar with the biology behind multiples.

The big picture: Fraternal twins develop when two eggs are fertilized in the same cycle, while identical twins develop when one fertilized egg splits into two embryos.

• Fraternal twins can run in families.

Between the lines: There's no single "twin gene."

• Instead, when fraternal twins keep showing up in a family without fertility treatment, the reason may be an inherited tendency to hyperovulate, or release more than one egg in a cycle.
• Scientists have linked a few common gene variants to that tendency, including some that involve a hormone that helps eggs mature.

Yes, but: Identical twins appear to turn up largely at "random," scientists have found.

• And researchers are still trying to understand what causes a fertilized egg to split.

The intrigue: Sylia Wilson, associate professor at U of M, recently encountered one family tree with three sets of identical twins.

• A woman enrolling in Minnesota's infant twin registry had just given birth to identical twins. Her sister also has identical twins, and their mother is an identical twin, Wilson says.
• Wilson cautions that the story is "anecdotal," but it raises questions about whether there may be a genetic component scientists don't yet understand.

We asked Axios Twin Cities readers to tell us about the twins in their families.

• Here's what they shared about their favorite or funniest memories.

💌 Great minds think alike. When Ellen Sundell-Madden and her twin sister play Pictionary on the same team, "we can draw just a few of the lines and we already know what is being drawn," she says.

• And a few times, Sundell-Madden and her twin have randomly bought the exact same birthday card for a family member.

🎂 Expect more cake. Paul Streeter's mother is an identical twin, and his fraternal twin siblings were born on his fourth birthday.

• Their mother always made three cakes to celebrate, he says.

💞 A "double blessing." That's how Angel Fitzgerald describes learning she was having twins — during delivery. "We have nine grandchildren at this point but no twins…so far!"

Our picks:

👭 Carly and her twin sister are both moms, but neither has twins…yet.