Dynamics of Return and Asymmetric Volatility Transmission Between the Stock Market and Iran’s Over-the-Counter Market: An Asymmetric Bivariate BEKK-GARCH Model
Keywords:
Volatility transmission, volatility failures, bivariate BEKK, GARCH model, leverage, stock market and OTC marketAbstract
Objective: This study examined the dynamic transmission of returns and asymmetric volatility between Iran’s stock market and over-the-counter market while jointly accounting for volatility breaks, thin trading, and aggregate trading volume.
Methodology: Daily observations for the Iranian stock market and over-the-counter market from September 28, 2009, to February 19, 2025, comprising 3,672 observations, were analyzed. The Iterated Cumulative Sum of Squares algorithm was applied to identify structural breaks in unconditional variance. A linear autoregressive state-space model with time-varying coefficients and the Kalman filter was used to adjust the return series for autocorrelation caused by thin trading. Return transmission, short- and long-run volatility spillovers, and asymmetric shock effects were estimated using a bivariate vector autoregressive model combined with an asymmetric BEKK-GARCH specification. Aggregate trading volume and dummy variables representing volatility breakpoints were subsequently incorporated into the mean and variance equations of the augmented model.
Findings: Unit-root tests indicated that stock-market returns, over-the-counter-market returns, and trading volume were stationary at level. Significant autoregressive coefficients confirmed return-memory effects and demonstrated that previous returns affected current returns in both markets. In the baseline specification, the asymmetry coefficients were statistically insignificant, indicating that positive and negative shocks exerted similar effects on conditional variance. However, after controlling for volatility breakpoints, aggregate trading volume, and thin-trading adjustments, the asymmetry coefficients became significant and leverage effects were confirmed in both markets. Negative shocks generated larger increases in volatility than positive shocks of a comparable magnitude. The results also supported significant dynamic return and volatility transmission and interdependence between the two markets.
Conclusion: Jointly incorporating volatility breaks, trading volume, and thin-trading adjustments provides a more accurate representation of cross-market return transmission and asymmetric volatility behavior. Omitting these factors may distort estimates of volatility persistence and underestimate the stronger response of both markets to adverse shocks.
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