  Most finance studies are discussed on the basis of several hypotheses, for
example, investors rationally optimize their investment strategies. However,
the hypotheses themselves are sometimes criticized. Market impacts, where
trades of investors can impact and change market prices, making optimization
impossible. In this study, we built an artificial market model by adding
technical analysis strategy agents searching one optimized parameter to a whole
simulation run to the prior model and investigated whether investors' inability
to accurately estimate market impacts in their optimizations leads to
optimization instability. In our results, the parameter of investment strategy
never converged to a specific value but continued to change. This means that
even if all other traders are fixed, only one investor will use backtesting to
optimize his/her strategy, which leads to the time evolution of market prices
becoming unstable. Optimization instability is one level higher than
"non-equilibrium of market prices." Therefore, the time evolution of market
prices produced by investment strategies having such unstable parameters is
highly unlikely to be predicted and have stable laws written by equations. This
nature makes us suspect that financial markets include the principle of natural
uniformity and indicates the difficulty of building an equation model
explaining the time evolution of prices.
