Aggregate demand curve, why?

   The economics syllabus in the DSE programme of Hong Kong requires textbooks and teachers to explain why aggregate demand (AD) curve is downward-sloping in macroeconomics. Furthermore, three reasons are cited: wealth effect, interest rate, and export.
   To me, explaining why AD curve is downward-sloping is not an easy task, at least more complicated than explaining demand curve in microeconomics. But what DSE requires is the opposite: demand in micro is unexplained but AD in macro must be explained.
   Even so, I support explaining to students for why AD takes certain shapes. As mentioned in my last post, scientists look for explanations. We should teach our students by taking this attitude seriously.
   It should be explained is one issue. How it should be explained is another issue. The suggested three reasons cited above, to me, are rather complicated, not easy for explanation, and even (potentially) problematic for some of them. In particular, I think wealth effect is not a straightforward concept. If you ask me to explain it (properly) to high-school students, I would find it a difficult task.
   To appreciate the difficulty involved in understanding wealth effect, one need only to know that this effect must be distinguished from the income effect that is mentioned in the last post. In other words, apart from income effect, there is another effect from price through wealth that affects the aggregate demand. This important point is perhaps not noticed by some textbook authors. I have read some books' explanation on wealth effect, which is essentially about income effect. But this is not so appropriate.
   Why it is problematic to confuse income effect with wealth effect? This is because, on aggregate, there is no income effect! Why does income effect reduce demand when prices of goods are higher? If one has a fixed income, but the prices of goods are higher, this person can buy fewer goods with the same income. But this is an individual's case and a microeconomic aspect. For the whole society, if prices are higher, although buyers cannot buy more, sellers selling more expensive goods earn more. In other words, some people's income is higher (not fixed), and they may buy more goods. Hence, while some people may buy less, some people may buy more. Taken as a whole, there need not always be income effect from prices.
   Thus, if we say prices on aggregate generate wealth effect, this effect must be something independent of the income effect conditional on a fixed income for everyone.
   If wealth effect is a straightforward concept, more university textbooks should perhaps be willing to mention it. The concept seems to appear often in newspapers or non-technical economics reports (perhaps that's the reason why DSE syllabus requires teachers to teach it). But some serious textbooks choose to avoid this concept. I haven't read all macroeconomics university textbooks. However, the two most popular ones, Gregory Mankiw's Intermediate Macroeconomics and Olivier Blanchard's Macroeconomics, do not cover the term.
   Although another textbook, Robert H. Frank and Ben S. Bernanke's Principles of Macroeconomics, has covered it, this book emphasizes intuitions instead of logical rigor, and so teaches in a style very much similar to DSE syllabus.
   Would logically rigorous textbooks cover it? Interestingly, I find older textbooks like William Branson's Macroeconomic Theory and Policy and Rudiger Dornbusch and Stanley Fischer's Macroeconomics do cover. Both books introduce the term in the context of the life-cycle consumption theory as introduced by Albert Ando and Franco Modignianli (Nobel prize winner). To require high-school students to understand the life-cycle theory is not appropriate. Even if they are able to understand it, I would rather teach them other topics in macroeconomics.
   Hence, what happen is: popular and rigorously written university textbooks avoid wealth effect. Meanwhile, a high-school syllabus explicitly suggest using it while avoiding the more intuitive explanation of law of demand in micro. I know there must be some (perhaps good) reasons behind this treatment. But if I can choose, I will not design the syllabus in this way.