Inflation outlook, rate differentials, projected growth, positioning, quants... there are countless explanations provided daily to explain why bonds trade the way they do. And yet, as Bank of America shows today, as of this moment just over 50% of the global bond market returns can be explained with just one thing: central bank balance sheet changes.

BofA explains:

Central bank assets, most of which are held in fixed income assets, are now equivalent to 31% of the $49tn fixed income universe tracked by the BofA Merrill Lynch Global Fixed Income Markets Index (GFIM); and the percentage of global bond market monthly returns explained by the monthly change in central bank balance sheets has dramatically increased in recent years.

And with more than half of bond returns now driven by central banks, BofA goes so far as to say that "Central banks have become the bond market."

BOfA's evidence:

Note how in the past year, the months in which central bank asset purchases have either declined or been very small have coincided with months of weak performance from global bonds (Table 2). This was particularly the case in the fourth quarter of last year and a similar pattern is emerging this summer.

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Of course, this is a problem because with central bank balance sheet projected to decline for the foreseeable future as Citi showed last month, at least until global stocks tumble and/or the next recession hits, it would suggest that yields have just one direction to go.