One of the reasons I keep a working diary while researching a book is because it is almost impossible otherwise to remember the moment when you discovered something crucial about your subject. By the time I am ready to hand in the manuscript, I feel that I have always known everything about this person, when in truth it was a slow process of discovery.

But on the subject of what mattered most to Ethel Rosenberg—which, after completing my biography of one half of the Soviet-spy couple, I know was her role as a mother—my diary was of little help.

Ethel does the dishes in her family’s New York apartment, 1950. She’d be arrested that same year.

Before starting my research, I might have believed the prevailing myth that, as a good Communist, Ethel would have placed the party and the communal good above the needs of individual family members. Where was the one critical moment when I realized that her life’s goal was not simply being a better mother than her mother had been—Tessie Greengrass had been a bitter, disappointed woman whose love and ambition were reserved for her sons—but being the best possible mother?

Was it the grand-jury revelation that she was worried about confusing the children by celebrating both Christmas and Hanukkah?

Was it the way Ethel got onto the floor to play with her challenging firstborn son rather than chat with a fellow mother, who, considering playdates an opportunity for adult gossip, concluded that Ethel was peculiar?

Or was it the decision to have a second child—Robby, born in May 1947—when life was difficult at home and Ethel and Julius Rosenberg had no certainty of material success? (Julius lost his job as a government inspector at the end of the war and decided to set up a small government-surplus hardware shop, but it never took off. Ethel was a full-time housewife and mother to Michael, then three.)

Bringing Up Babies

By the 1950s there was a growing belief that women could aspire to become perfect wives and mothers, fostered by advertisers peddling new fridges and vacuum cleaners galore.

Ethel did not have the wherewithal to fall prey to this materialism but instead paid for child-rearing classes for her two sons as part of a highly theoretical course in child psychology entitled “The Child from Birth to Six Years,” which met once a week for two hours at the progressive and extremely prestigious New School for Social Research (N.S.S.R.) and was taught by the Viennese émigré Dr. Edith Buxbaum, who had studied with Anna Freud.

Ethel, as a young mother, looking intently at her toddler, Michael, on South Beach. Michael recalls that they would often take the ferry and then the Staten Island Rapid Transit to a beach on Lower New York Bay.

Increasingly, the new science of child-rearing appeared as a perfectible art, thanks to books such as Dr. Spock’s Baby and Child Care and Dr. Dorothy Whipple’s Our American Babies. Whipple also had an influential column in Parents magazine, which Ethel devoured.

One of the most tragic moments in the Rosenberg trial was when Julius, asked about his wife’s clothes, replied that Ethel had spent at most about $300 on her wardrobe over the previous 10 years. Looking at her drab outfits in the courtroom, the jury would have had little difficulty in believing it.

Ethel did not fall prey to 1950s materialism but instead paid for child-rearing classes for her two sons.

Ethel cared so little about her clothes that she did not realize the importance of looking good in court. What consumed her was being a good mother. Yet the judge at her trial, which took place in New York 70 years ago, in sentencing her to death for conspiracy to commit espionage, accused her of showing greater love for her cause than for her children. For Ethel, the judge’s stinging criticism of her as a parent was the cruelest barb of all.

The Sing Sing electric chair that the Rosenbergs were executed in.

Understanding why being a good mother mattered so much to Ethel was a cumulative process, which is why I did not specify a single moment in my diary. But perhaps there was one after all: a photo I had never seen before, which Ethel’s sons gave me permission to look at. It shows a mother staring intently into her baby’s face as if there she might discover the miracle that has given him life. Ethel, probably not a natural mother, had discovered the magic of motherhood.

Once I understood this, I had great insight into Ethel’s final predicament. She decided that the best legacy she could leave her children was not her own physical presence—however much she longed to see her then 6- and 10-year-olds grow up—but an example of how to behave with dignity in the face of fear. She was electrocuted on June 19, 1953, choosing death rather than the betrayal of her husband, her children’s father.

Anne Sebba’s Ethel Rosenberg: An American Tragedy is out now from St. Martin’s