It’s also a distinction that falls apart when you think too much about it.
Is it about how much math is used? Because social science can involve a shit-ton of hard math (check out computational social science), and biology, despite becoming “softer” over time (as @anaisnein notes), is more maths & CS heavy than ever in all specialties (judging from all my PhD buddies).
Is it about how physically tangible the concepts are? Because I’m not sure dark matter qualifies, and a lot of what we know about quantum mechanics was largely theoretical & unproven for years, and we still don’t have a very intuitive understanding of how some of it works. We’ve just had longer to work on these problems than problems from other disciplines, but that’s a poor metric for “hardness.”
Is it about the steepness of the learning curve? Maybe, because sure, psychology 101 and sociology 101 are more intuitive & easy to grasp than physics 101 - we use normal words that normal people know, & most people in these classes are humans, so they have, you know, experience with these concepts - but when people are making these comparisons, they’re often mentally comparing psych 101 to work at CERN, and that’s…just not a fair comparison. Start imagining all of the working parts you need to pin down from psychology, sociology, public health, education, neuroscience, and biology in order to understand human behavior, & then imagine all of the work in computer science, data science, and statistics you need to learn in order model & test this behavior. There’s a reason I refer to myself as a social scientist rather than anything more specific - studying humans well requires a vast amount of knowledge from a large number of disciplines. Let me know how long you think that’ll take you to master…
Is it about the importance of the problems? I’ve heard physicist revel in the fact that they are investigating the fundamental fabric of our universe. And sure, that’s super awesome!! I love physics findings!! But are you telling me you don’t also care about the health of our planet? Or what’s living in our oceans? Or how to cure diseases? Or how are genes work? Or how to stop crime? Or how to better educate our children? Or how to live a more fulfilling life? Or how to prevent or treat mental illness? Sure, the universe will outlive us, but you have to live on this planet now, and your kids and your grandkids and your great-grandkids have to live on this planet in the future. Are you telling me that investigating problems that may make yours or their lives better is not of comparable interest to you??
What’s left? Perhaps the degree to which you can bullshit through the degree without knowing what you are doing? I might give you this one. In undergrad, you would’ve been hard-pressed to fake your way through our chemistry program, but you might’ve eked by in our psych program and probably could’ve done so in our soc program. But you’re not going to be hired in that field doing that job. The people who call themselves “social scientists” are not those people, not it’s a poor way to categorize disciplines.
So tell me…what exactly is a hard or soft science again?