According to Eric Balchunas, the US equity market has underperformed compared to global markets only once in the past 16 years, specifically in 2017. Current trends suggest 2023 may become the second instance, with early indicators showing consistent underperformance. This observation is critical for traders focusing on inter-market analysis and considering portfolio diversification strategies to mitigate risks associated with US equities. Source: @SirYappityyapp.