我在读一篇文献的时候看到一句话
A country's current account will be in deficit whenever national cash flow, defined as output (gross domestic product, or GDP)less investment less government spending, is expected to rise over time. It will be in surplus whenever national cash flow is expected to fall over time.
其中national cash flow定义为（Y-I-G）为什么这个值上升，经常性账户将会是赤字呢？
反过来为什么这个值下降，为啥是顺差？