  Living organisms, ecosystems, and social systems are examples of complex
systems in which robustness against inclusion of new elements is an essential
feature. A recently proposed simple model has revealed a general mechanism by
which such systems can become robust against inclusion of elements with random
interactions when the elements have a moderate number of links. This happens as
a result of two opposing effects such that while the inclusion of elements with
more interactions makes each individual element more robust against
disturbances, it also increases the net impact of the loss of any element in
the system. The interaction is, however, in many systems often intrinsically
bidirectional like for mutual symbiosis, competition in ecology, and the
action-reaction law of Newtonian mechanics, etc. This study reports the strong
reinforcement effect of the bidirectionality of the interactions on the
robustness of evolving systems. We show that the system with purely
bidirectional interactions can grow with two-fold average degree, in comparison
with the purely unidirectional system. This drastic shift of the transition
point comes from the reinforcement of each node, not from a change in structure
of the emergent system. For systems with partially bidirectional interactions
we find that the area of the growing phase gets expanded. In the dense
interaction regime, there exists an optimum proportion of bidirectional
interactions for the growth rate at around $1/3$. In the sparsely connected
systems, small but finite fraction of bidirectional links can change the
system's behaviour from non-growing to growing behaviour.

Adaptation and Self-Organizing Systems;Statistical Mechanics