The Cost of Modularity 267
15.5 Kinds of Costs of Modularity
While the cost side is missing in current explanations of the evolution of nearly decompos-
able systems, cost-benefi t analysis was present from the very beginning of the modularity
debate. It was already part of Herbert Simon’s famous watchmaker metaphor from 1962:
Two watchmakers, Hora and Tempus, both built their watches from 1,000 parts. Tempus’s
watch was highly integrated, while Hora’s watch was modular, consisting of stable subas-
semblies of 10 parts each. Both watchmakers were frequently disturbed by telephone calls.
To accept a call, they had to put down the assembly they are working on, which then fell
apart. Tempus hardly ever fi nished a watch and became poorer and poorer while Hora
prospered (Simon 1969: 188).
The reason for the different success of Hora and Tempus is that Tempus, on an inter-
ruption by a telephone call, loses the time for up to 999 assembly steps. Hora, in contrast,
loses on no single call more time than needed for 9 steps, because every tenth step yields
a stable assembly.^11 This is the benefi t of modularity. But, in contrast to the biological
arguments mentioned, Simon also considers the costs. Hora needs more steps to fi nish a
watch: he has to complete 111 subassemblies and needs a total of 1,110 steps. Tempus
needs only 1,000 steps. So the cost of modularity is an additional 11 percent of work.^12
What happens if we disregard, in contrast to Simon, the cost side and focus on the benefi t
alone? We could then maximize the estimated benefi t of modularity by making the modules
smaller and smaller. In the extreme, every module may consist of only 2 parts. This would
minimize the loss of work on interruptions by telephone calls. But there are in fact addi-
tional costs of such a strong modularization so that in the end it would not pay off. A
watchmaker—let his name be Minuta—who applied such extreme modular design needed
1,000 + 500 + 250 +... + 1 = about 1,999 steps to fi nish a watch.^13 So Minuta needed
almost twice as many different steps as Tempus to assemble a watch, which might be
worth his while only under extreme phone harassment.
Additional kinds of costs may be associated with modularity. I therefore describe dif-
ferent classes of costs as they can be derived from descriptions of modular design in
technical artifacts, and apply them to the fi eld of biology.
- The aforementioned costs of additional assembly steps that must be performed fi nd
their equivalent in biology in the extended time required for ontogenesis. The ontogenesis
of a modularized organism, according to these considerations, needs more time than the
ontogenesis of a higher integrated organism.^14 The costs of a longer ontogenetic process
can be seen in higher energy requirements for development and a higher risk of dying
before offspring are produced.
2 and 3) Keeping a region of a network that is singled out as a module working entails
additional energetic effort and need for material. The module is nearly decoupled from the
rest of the network. In engineering, it is well known that a modular organization thus
causes higher weight and volume and material effort (Pahl et al. 2007: 509). This can be