It’s hard to believe that 40 years have passed since Kramer vs Kramer was a cultural phenomenon, a conversation-starter that grossed more than any other movie in 1979 and then swept the Oscars four months later, winning best picture along with prizes for both Kramers, Dustin Hoffman and Meryl Streep, and two awards for Robert Benton for his adapted screenplay and direction. And yet there are aspects of the film that seem older still, as if it were some artifact from a culture that’s utterly foreign and incomprehensible. The judgment rendered against Ted Kramer in divorce court – and the shockingly odious terms of his child visitation rights – is so unjust that the film could be interpreted as Men’s Rights propaganda.

That interpretation would be wrong, for many reasons, but it’s worth noting that legal experts at the time were puzzled by it, too. In fact, in a New York Times survey of four jurists the week the film came out, Felice K Shea, then acting justice of the New York State supreme court, lamented, “It’s too bad that the legal profession was portrayed as 50 years behind the times … In so many ways the court process was portrayed as unfeeling, unrealistic and incomplete.” And so by that math, Kramer vs Kramer seems nearly a century old, ending with a custody judgment and a denouement that are baffling for two loving parents who live in the same city.

Photograph: Allstar/Columbia

Yet Kramer vs Kramer remains a film of immense sensitivity and heart, and context helps clear up some confusion about the film’s depiction of parenthood, gender roles and the process of getting a divorce, all of which were closely intertwined and in a state of flux. When the film came out, it had only been 10 years since California became the first state to pass a no-fault divorce law, in which the dissolution of a marriage does not require evidence of wrongdoing by either party. And New York, where the film takes place, was the last of the 50 states to allow no-fault divorce, through a law that wasn’t signed until 2010.

Broadly speaking, no-fault allowed for easier and cheaper divorces, which both unburdened the legal system and accommodated a higher divorce rate. While the law isn’t specifically relevant to Kramer vs Kramer – which has the parties fighting in court at great expense – it does reflect a change in the culture, where women especially were better able to leave unhappy marriages and achieve self-actualization. That didn’t relieve the stigma of a mother leaving a child behind, as Joanna Kramer (Streep) does here, or undercut social biases about women being better caregivers to their children than men, which ultimately harms Ted Kramer (Hoffman) in court. But when the film came out, such matters were up for fierce debate.

Viewers are definitely free to assign fault to the characters in Kramer vs Kramer, just as they currently are over Adam Driver and Scarlett Johansson’s characters in Marriage Story, which has an equally jaundiced view of the legal process. And at first glance – and maybe every glance, for some – Benton’s film is hugely unbalanced in favor of Ted, a workaholic ad executive who scrambles to look after his son Billy (Justin Henry) when Joanna quietly yet firmly walks out the door. Joanna only appears near the beginning and end of the film, and in the 18 months between when she leaves and when she comes back seeking sole custody of Billy, all we see are scenes of father and son bonding together. In terms of audience sympathy, she never stands a chance.

But Benton and Streep do their best with the time that they have. Kramer vs Kramer opens with a devastating scene of Joanna tucking Billy into bed one last time, stroking his hair and telling the boy she loves him. Had the film been about her journey, it might have been something like Francis Ford Coppola’s excellent 1969 road movie The Rain People, in which a housewife leaves her husband for destinations unknown, motivated equally by wanderlust and a feeling of suffocation at home. Joanna is obviously a deeply unhappy woman, forced to cede her ambitions to her career-driven husband and beaten down by motherhood, with all its hassles and loneliness. Streep suggests a combination of vulnerability and resolve, as if Joanna is using the vapors of self-esteem she has left to will a new life for herself.

And it’s clear from the start that Ted has been clinging to old-fashioned notions of parenthood, when men were married to the office and fatherhood was more of a ceremonial role. The morning after Joanna leaves, he disastrously improvises a breakfast of French toast and coffee, and loses his temper around his son, not for the last time. The 80s would make comedies like Mr Mom and Three Men and a Baby out of hapless fathers thrown into domestic chaos, but Kramer vs Kramer plays it straight, as the steep learning curve caused by his negligence. If the Kramers’ custody battle came down to hours logged, Ted wouldn’t stand a chance, even counting 18 months his wife was gone completely.

Photograph: RONALD GRANT

For as difficult as Kramer vs Kramer can be to accept as a film about divorce – both from a legal standpoint and from our imbalance of sympathies for the Kramer – it’s often exceptionally beautiful as a story about a father learning to have a real relationship with his son. Ted doesn’t know how to set boundaries at first, so Billy tests them in a great scene where he refuses to eat the umpteenth TV dinner and goes straight for the ice cream in the freezer, daring his father to follow through on a threat to punish him. All Ted can do is yell at the kid impotently (“I hate you, you little shit!”), as bad parents do, because their kids have no idea what to expect from them.

Once the two find a groove, the film picks up on the ease they start to feel around each other as they settle into a routine and Ted prioritizes his son over the biggest account of his career. Whenever the action shifts outside, the legendary French cinematographer Nestor Almendros, a master of wide-angle lenses and natural light, seems to pump the film full of oxygen, as if they can finally breathe together on their own power. When one of those playground trips ends in an incident where Billy nearly loses an eye, Ted’s response in the emergency room – advocating for his son while staying by his side for every stitch – is not something he’d have been capable of doing before. Squaring up to his responsibilities makes him a better person.

The ending of Kramer vs Kramer is a terrible copout, a Hollywood reversal of the gross injustice doled out by the court. By that point in the film, Ted has a better understanding of Joanna than he did at the beginning, because now he knows what her day-to-day life was actually like and he’s come to terms with his own screwed-up priorities. For Joanna to forfeit custody entirely is supposed to redeem her, but it makes her the villain once more, because she’s walking out on her son a second time. Even 40 years ago, parenthood wasn’t a zero-sum game.