Based on election results in the modern era, the assertion would seem to be unassailable. Since the end of WWII there have been 34 federal elections in the United States. In 22 of those elections, Americans elected a divided government. We chose divided government for 44 of the intervening 71 years or 62% of the time. We are currently in the 6th consecutive year of our most recent iteration with indicators pointing to divided government extending it's winning streak for at least two and probably four more years.
Wait. Do We Do That On Purpose?
That is the question. It's a simple fact that we vote for divided government far more often than not. It's that "cognitive" part that political scientists and pundits have a hard time swallowing. They just can't believe we are doing this deliberately.