The shallow-lakes analogy
What Scheffer's lakes can teach us about depression, and about the limits of thinking in causes.
One of the most illuminating analogies from the work of Marten Scheffer concerns the shallow lakes of the Netherlands. Many of these lakes are turbid, polluted and poor in plant and animal life. But that need not be the case: the same lake can also be clear, with a varied fish population, water plants and visibility down to the bottom. What determines which state a lake is in? And, more important for practice: how can a turbid lake be made clear again?
The obvious line of reasoning is that the amount of phosphorus entering the lake is decisive. A great deal of phosphorus, from agriculture and wastewater, leads to algal bloom, which disturbs the ecosystem and makes the water turbid. Lower the phosphorus load, so the thinking goes, and the lake will become clear again of its own accord.
When this was seriously tried a few decades ago in various places in Europe and beyond, it turned out not to work. The phosphorus load was brought down to well below the level at which the lakes had once still been clear, and yet the clarity did not return.
Hysteresis: there and back are not the same
Ecologists discovered that such a lake behaves like a complex system with two stable states, clear and turbid, and with a hysteresis effect between those states. The tipping point from clear to turbid lies at a much higher phosphorus load than the point at which the lake tips back. That return point may even lie so low that it is in practice no longer attainable in an urbanised landscape.
Thinking in linear dose-response relations (more phosphorus, more turbid water; less phosphorus, clearer water) falls short here. The lake is not a linear system but a complex system with two attractors. Once it has ended up in the turbid attractor, more is needed than removing the cause to get out of it.
"The dogma of intervention, that the cause of the problem is the key to the solution, does not necessarily apply to complex systems." (Han van der Maas, after Marten Scheffer)
What did work: catching fish
The breakthrough came in the 1980s, when ecologists took a far-reaching measure: in winter they netted almost all the fish out. At first sight this seems to make little sense, for what does fish have to do with phosphorus? Yet it worked. In spring a new equilibrium arose, with other fish species, water plants and clear water, and that clear state often proved stable, sometimes for many years.
The cause, phosphorus, thus turned out not to be the key to the solution. That key lay in an intervention on an entirely different variable, at a different level of the system, which moved the lake far enough out of its equilibrium to tip into the other attractor.
From lakes to depression
In his book, Han van der Maas explicitly extends this parallel to depression. A depression, too, has much in common with a turbid lake. There appear to be two stable states, healthy and depressed. The tipping can come suddenly, often after a stressor that is the proverbial last straw. And then a strikingly persistent phenomenon follows: the stressor disappears, life improves, and yet the depression remains. The way back does not follow the same route as the way there.
That removing the original stressor does not automatically lead to recovery therefore need not point to a failed treatment. It is consistent with the properties of complex systems with hysteresis. It may therefore be worthwhile to look beyond the cause and to search for the "fish" that we can catch in order to break through the attractor.
What is the "fish" in depression?
As far as we know, there is as yet no unequivocal answer to that question. The idea itself is nonetheless useful: which interventions, which at first sight have little to do with the original cause of a depression, might still set the system sufficiently in motion to make a tipping possible?
A few candidates, with varying strength of evidence:
- Behavioural activation: not waiting until someone feels better before doing things, but doing things in order to be able to start feeling better. An intervention on behaviour, not on the "cause".
- Restoring sleep: often an early and noticeable tipping moment, even when the original stressor had nothing to do with it.
- Social reconnection: not so much as "treatment" of the depression, but as a change in the context in which the depression maintains itself.
- Exercise: strikingly effective, with effects that appear to reach down to the neurophysiological level.
- A clearly different environment: an admission, a retreat, a stay abroad. Sometimes an interruption of routine seems to be precisely what the system needs.
None of these routes offers guarantees. Still, it seems to us a more fruitful line of thought than trying again and again to remove "the cause". For patients who have been told for years that things will improve once they work on their trauma, while the trauma does not yield and the depression remains, this way of looking may come as a relief. That the linear model falls short in their situation is not their doing; the system works differently.
Inspiration for the consulting room
The shallow-lakes analogy invites a different way of looking at a patient who is stuck. In the first place, it invites some restraint towards the self-evident idea that cause and solution are each other's mirror image. Sometimes that is so, sometimes not at all. It is a hypothesis, not a given.
In addition, it invites creativity in the choice of interventions. Which "fish" might be caught here? Which variables in this life were perhaps not "the cause", but could still set the system in motion, or help it to reorganise itself?
And finally, it invites patience with patients who have been in their "turbid" state for years. That need not point to unwillingness or weakness. It may also reflect the persistence of an attractor, waiting for the right moment, the right intervention or a favourable conjunction of circumstances.
The analogy does not, of course, explain everything. But once the image is in view, many clinical experiences can be placed more clearly. It is a metaphor with a scientific foundation, and it is precisely that combination that makes it useful.