probability of which is quadratically proportional to their current shares. A stochastic model of such biquadratic transfer of discretized capital among products is proposed, which reproduces the empirical distribution and evolution of product share, confirming that the shifts of investment strategy are made mostly referring to the current shares of products. Random transfer models have been similarly proposed to explain the distribution of wealth among individuals or countries [@yakovenko2009colloquium; @bouchaud2000wealth]. The functional form of the transfer rate determines whether the system is either in the fluid phase or the condensate phase with the biquadratic one at the boundary. Analytic solutions and simulation results reveal the possibility of condensation of capital onto few key products and predict its time scale. Therefore the studied model can be used as a framework helping understand and analyze the overwhelming complexity of international trade.

Distribution and annual variation of product share {#sec:empirical}
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![ Product share in international trade. (a) Probability density function of product share $A$. Empirical data (symbols) for year 1962 are fitted by a lognormal function $P(A)\sim A^{-1} \exp\left[-{(\log A - \mu)^2 \over 2\sigma^2 }\right]$with $\mu=-7.1$ and $\sigma=1.3$ (dashed line) [@doi:10.1137/070710111; @Alstott:2014aa]. The solid line is taken from model simulations at the 