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The Biggest Reason to Be Thankful for Your Brothers and Sisters on Siblings Day

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Siblings Cover
The July 10, 2006, cover of TIMECover Credit: PHOTO-ILLUSTRATION FOR TIME BY ARTHUR HOCHSTEIN. PHOTOGRAPH OF CHILDREN BY PENNY GENTIEU - BABYSTOCK.COM. NEST FROM GETTY IMAGES
By Lily Rothman

Our brothers and sisters are often the only people in our lives from beginning to end, and their influence is just as broad as that timeline.

That was the conclusion reached by TIME’s Jeffrey Kluger in a 2006 cover story about the effects of sibling-hood. And, as many U.S. states mark Siblings Day on April 10, there’s no better time to look at that research as another reason to be thankful for those closest of relatives. Even the ever-present sibling rivalry can pay off in the long run, it turns out, as they help kids grow up with problem-solving know-how.

Read TIME’s new special report on siblings: How Parents Can Help Their Kids Get Along

And, as Kluger reported, it’s not really surprising that those childhood companions end up affecting our adulthood:

From the time they are born, our brothers and sisters are our collaborators and co-conspirators, our role models and cautionary tales. They are our scolds, protectors, goads, tormentors, playmates, counselors, sources of envy, objects of pride. They teach us how to resolve conflicts and how not to; how to conduct friendships and when to walk away from them. Sisters teach brothers about the mysteries of girls; brothers teach sisters about the puzzle of boys. Our spouses arrive comparatively late in our lives; our parents eventually leave us. Our siblings may be the only people we’ll ever know who truly qualify as partners for life. “Siblings,” says family sociologist Katherine Conger of the University of California, Davis, “are with us for the whole journey.”

Within the scientific community, siblings have not been wholly ignored, but research has been limited mostly to discussions of birth order. Older sibs were said to be strivers; younger ones rebels; middle kids the lost souls. The stereotypes were broad, if not entirely untrue, and there the discussion mostly ended.

But all that’s changing. At research centers in the U.S., Canada, Europe and elsewhere, investigators are launching a wealth of new studies into the sibling dynamic, looking at ways brothers and sisters steer one another into–or away from–risky behavior; how they form a protective buffer against family upheaval; how they educate one another about the opposite sex; how all siblings compete for family recognition and come to terms–or blows–over such impossibly charged issues as parental favoritism.

Still, folks without siblings shouldn’t despair: All that research into siblings came along with research into only-children. The stereotype of the spoiled only child, scientists found, was mostly just a myth.

Read the full story, here in the TIME Vault: The New Science of Siblings

Read next: Facebook Reveals How Common It Is For Siblings to Have The Same First Initial

Sister Act: Joan Fontaine and Olivia de Havilland

As successful movie stars, Joan (left) and Olivia make one of their rare appearances together in a window at Joan's home. This year they were winner and runner-up, respectively, for the Academy Award.
Caption from LIFE. As successful movie stars, Joan (left) and Olivia make one of their rare appearances together in a window at Joan's home. This year they were winner and runner-up, respectively, for the Academy Award.Bob Landry—The LIFE Picture Collection/Getty Images
Joan Fontaine in her film make-up and pompadour.
Caption from LIFE. Joan Fontaine in her film make-up and pompadour.Bob Landry—The LIFE Picture Collection/Getty Images
Olivia, here sunk in a brown study, has changed considerably. The cheery but colorless Hoopskirt Girl has given way to an understandably gloomier but much greater actress.
Caption from LIFE. Olivia, here sunk in a brown study, has changed considerably. The cheery but colorless Hoopskirt Girl has given way to an understandably gloomier but much greater actress.Bob Landry—The LIFE Picture Collection/Getty Images
Joan Fontaine in natural freckles and pigtails looks very different but no less pretty and fragile than she does in her film make-up and pompadour.
Caption from LIFE. Joan Fontaine in natural freckles and pigtails looks very different but no less pretty and fragile than she does in her film make-up and pompadour.Bob Landry—The LIFE Picture Collection/Getty Images
On the back terrace of their white Georgian home in Beverly Hills, Joan and her husband, Actor Brian Aherne, chat with Olivia. The Ahernes are extremely happy and domestic, like to have tea each afternoon at 4, to dress for dinner and have in Aherne's British friends. He is a licensed pilot and now helps in the West Coast Civil Air Patrol.
Caption from LIFE. On the back terrace of their white Georgian home in Beverly Hills, Joan and her husband, Actor Brian Aherne, chat with Olivia. The Ahernes are extremely happy and domestic, like to have tea each afternoon at 4, to dress for dinner and have in Aherne's British friends. He is a licensed pilot and now helps in the West Coast Civil Air Patrol.Bob Landry—The LIFE Picture Collection/Getty Images
Her gold Oscar Statuette stands on the desk where Joan attends to the bookkeeping for the Aherne household. Maps for their frequent airplane trips cover the study walls.
Caption from LIFE. Her gold Oscar Statuette stands on the desk where Joan attends to the bookkeeping for the Aherne household. Maps for their frequent airplane trips cover the study walls.Bob Landry—The LIFE Picture Collection/Getty Images
Olivia De Havilland, 1942
Olivia De Havilland, 1942.Bob Landry—The LIFE Picture Collection/Getty Images
Joan Fontaine with her husband Brian Aherne and her dog, 1942.
Joan Fontaine with her husband Brian Aherne and her dog, 1942.Bob Landry—The LIFE Picture Collection/Getty Images
Joan Fontaine, 1942
Joan Fontaine, 1942.Bob Landry—The LIFE Picture Collection/Getty Images
Olivia De Havilland, 1942
Olivia De Havilland, 1942.Bob Landry—The LIFE Picture Collection/Getty Images
Joan Fontaine, 1942
Joan Fontaine, 1942.Bob Landry—The LIFE Picture Collection/Getty Images

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Write to Lily Rothman at lily.rothman@time.com