Stocks don't change directly; they grow or shrink over time according to the rates of flow. If your salary grows while spending stays constant, money accumulates in your account. Flows are affected by changes to the rules of the system, and sometimes by the levels in each stock. The more money in your bank account, the faster you spend it, perhaps. When a flow affects a stock and that stock affects the flow, a feedback loop happens, and then things get interesting. If the more money in your bank account, the more you invest, and then the more money in your bank account... $$$$$!
This is a reinforcing feedback loop, marked with an R. It leads to accelerating growth, or accelerating decline.
Enough about money, let's talk about Women in IT. We can model this. The stock is all the women programmers working today. They come in through a pipeline, and leave by retiring.
People also leave programming because there is some other, compelling thing for them to do. This includes raising a family, writing a novel, etc. I'll call this the "Something else" flow. It should be about the same rate as in other lines of work.
Then there are women who drain out of IT because they feel drained. They're tired of being the only woman on the team, tired of harassment or exclusion, tired of being passed over for promotion. Threatened until they flee their homes, in some cases. Exhausted from years of microaggressions, in more. I'll call it the "F___ This" outflow.
There are feedback loops in this system. The more women in IT, the more young women see themselves as possible programmers, the more enter the pipeline. Therefore, the flow of the input pipeline depends on the level of the stock. This is a reinforcing feedback loop: the more already in, the more come in. The fewer already in, the fewer enter.
The quantity of women programmers also affects the experience for each of us. At any given conference or company, a few assholes are mean, and those microaggressions or harassments hit the women present. The fewer women, the more bullshit each of us gets. Also the less support we have when we complain. This is also a reinforcing feedback loop: the lower the number of women in IT, the more will say "F___ This" and leave. If there are ever as many women programmers as men, that outflow might be the same as in other fields. Until then, the outflow gets bigger as the stock gets lower.
One of my friends pointed at the pipeline and said, "It's not hopeless, because women are a renewable resource." He hopes we can overwhelm the outflows with a constant influx of entry-level developers. In Thinking in Systems, Meadows remarks that people have a tendency to focus on inflows when they want to change a system. Sure enough, there's a lot of focus these days, a lot of money and effort, toward growing the input pipeline. Teach young girls programming. Add new input pipelines: women-only bootcamps, or programming for moms. These are useful efforts. Yet as Meadows points out, the outflows have just as much effect on the stock.
Can we plug the holes in this bathtub? Everyone in IT affects the environment in IT. We can all help close the "F___ This" outflow. See Kat Hagan's list of ways we can each make this environment less sexist. Maybe in a generation it'll get better?
This isn't new information. Let's add granularity to the system, and see what we learn. Divide the stock into two stocks: junior and senior women developers. The pipeline can only provide junior developers. Experience takes them to senior level, and finally retirement. Junior people are likely to find other careers, while senior people have been here long enough we're probably staying so I won't draw that as an outflow. The "F___ This" outflow definitely exists for both.
Consider now the effects of the two stocks on the flows. The senior developers are most impactful on all of them - on the pipeline as teachers, on "F___ This" as influencers in the organization, and even on the "Something else" outflow as role models! The more women architects and conference speakers and book authors and bloggers a junior dev sees, the more she sees a future for herself in this career. Finally, the stock of senior women impacts the rate at which younger women move into senior roles, through mentorship and by impacting expectations in the organization and community.
To sustainably change the stocks in a system, work with the feedback loops. In this case, the senior developer stock impacts flows the most. To get faster results that outlast the flows of money into women-only bootcamps, it's senior developers we need the most. And the senior-er the better. Zoom in on this box, and some suggestions emerge.
Inflows1. Mentor women developers. Don't make her wait for another woman to do it; she may wait forever, or leave first. Overcome the awkward.
2. Consider minorities carefully for promotion. A woman is rarely the obvious choice for architect or tech lead, so use conscious thought to overcome cognitive bias.
Outflows1. Encourage senior women to stay past minimum retirement age. Our society doesn't value older women as much as older men. Can we defy that in our community?
2. Don't tolerate people who are mean, especially to women who are visibly senior in the community. See Cate Huston's post: don't be a bystander to cruelty or sexism. And above all, when a woman tells you something happened, believe her. For every one who speaks up, n silently walked away.
Magnify the impact1. Look for women to speak at your conference. Ask early, pay their travel, do what it takes to help us help the community.
2. Retweet women, repost blogs, amplify. I want every woman in development to know that this is a career she can enjoy forever.
Stocks don't change instantaneously. Flows can. It takes time for culture to improve, and time for that improvement to show up in the numbers. With reinforcing feedback loops like this, waiting won't fix anything - until we change the flows. Then time will be on our side.
Finally, please don't take a handful of counterexamples as proof that the existing system is fine. Yes, we all respect Grace Hopper. Yes, there are a few of us with unusual personalities who aren't affected by the obstacles most encounter. Your bank account is in the black with just a few pennies, but that won't make you rich.
 Thinking in Systems, by Donella Meadows. On Amazon or maybe in pdf
 Honestly women have it easier than a lot of other minorities, but I can only speak from any experience about women. Please steal these ideas if you can use them.
 There's a small flow into Women in IT from the stock of Men in IT, which is awesome and welcome and you are fantastic.
 Cate Huston talked about "how dismal the numbers were, and how the numbers were bad because the experience was bad, and how the numbers wouldn’t change unless the experience changed" in her post on Corporate Feminism.
 Anita Sarkeesian on "the most radical thing you can do to help: actually believe women when they talk about their experiences."