One of the things I'm surprised your article doesn't mention is the tax advantages of this type of investment. The depreciation and rehab costs (purchasing distressed properties) can be huge deductions to ones income taxes, which none of the others have. Then, along with the appreciation of real estate, this passive income investment outperforms the notion of maxing out my 401k as well.
Under British rule, India's share of the world economy declined from 24.4% in 1700 down to 4.2% in 1950. India's GDP (PPP) per capita was stagnant during the Mughal Empire and began to decline prior to the onset of British rule.[103] India's share of global industrial output declined from 25% in 1750 down to 2% in 1900.[78] At the same time, the United Kingdom's share of the world economy rose from 2.9% in 1700 up to 9% in 1870. The British East India Company, following their conquest of Bengal in 1757, had forced open the large Indian market to British goods, which could be sold in India without tariffs or duties, compared to local Indian producers who were heavily taxed, while in Britain protectionist policies such as bans and high tariffs were implemented to restrict Indian textiles from being sold there, whereas raw cotton was imported from India without tariffs to British factories which manufactured textiles from Indian cotton and sold them back to the Indian market. British economic policies gave them a monopoly over India's large market and cotton resources.[104][105][106] India served as both a significant supplier of raw goods to British manufacturers and a large captive market for British manufactured goods.[107]

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I wish I had more time to put into real estate. Given the run up since 2012, I may even be interested in selling my condo that I currently rent out. I need to get it appraised to really see what it’s worth, but I think conservatively it’s gone up ~50%, although rent is probably only up ~10% or so. I am bullish on rents going up in the future… mostly in line with inflation, or perhaps even slightly faster due to constricted credit and personal income growth which should provide a solid supply of renters. At this point, I just don’t want to manage the property. I’ll probably look into a property manager as my time is likely worth turning it into a nearly passive investment.
Industry accounts for 26% of GDP and employs 22% of the total workforce.[165] According to the World Bank, India's industrial manufacturing GDP output in 2015 was 6th largest in the world on current US dollar basis ($559 billion),[166] and 9th largest on inflation-adjusted constant 2005 US dollar basis ($197.1 billion).[167] The industrial sector underwent significant changes due to the 1991 economic reforms, which removed import restrictions, brought in foreign competition, led to the privatisation of certain government-owned public-sector industries, liberalised the foreign direct investment (FDI) regime,[168] improved infrastructure and led to an expansion in the production of fast-moving consumer goods.[169] Post-liberalisation, the Indian private sector was faced with increasing domestic and foreign competition, including the threat of cheaper Chinese imports. It has since handled the change by squeezing costs, revamping management, and relying on cheap labour and new technology. However, this has also reduced employment generation, even among smaller manufacturers who previously relied on labour-intensive processes.[170]
But, you don't need to go further than that. You can simply write it and publish it and collect the income. That's all. Send out a couple emails to your list (if you have one) or post it on social media, and there you have it. Passive income. Now, the amount of income you receive depends on the quality of the book you've written. How well did you craft the message? How targeted was the information to your audience? It counts.
Developing passive income is different. With the exception of one of my passive income streams (cryptocurrency mining), all of the others require real, hard work. Truly, I understand the barriers for people getting into building alternative income streams. I would say that most people WANT passive income, but truly aren’t willing to put in the blood, sweat, and tears to make it happen.
However, this comes back to the old discussion of pain versus pleasure. We will always do more to avoid pain than we will to gain pleasure. When our backs are against the wall, we act. When they're not, we relax. The truth is that the pain-versus-pleasure paradigm only operates in the short term. We'll only avoid pain in the here and now. Often not in the long term.
Estimated GDP per capita of India and United Kingdom during 1700–1950 in 1990 US$ according to Maddison.[97] However, Maddison's estimates for 18th-century India have been criticized as gross underestimates,[98] Bairoch estimates India had a higher GDP per capita in the 18th century,[99][100] and Parthasarathi's findings show higher real wages in 18th-century Bengal and Mysore.[101][78] But there is consensus that India's per capita GDP and income stagnated during the colonial era, starting in the late 18th century.[102]
I've now only got an SF rental condo and a Lake Tahoe vacation rental in my real-estate-rental portfolio. Although I miss my old house, I certainly don't miss paying $23,000 a year in property taxes and another mortgage, and dealing with leaks and managing terrible tenants. I drove by the other day and couldn't believe how much noisier and busier the street was than where I currently live. I wouldn't be comfortable raising my son there.

Capital growth in your portfolio can offset the eroding effect of inflation.  But any capital gains that exceed the overall inflationary effect can be drawn off to augment your portfolio income.  Of course, you must first generate those gains by making thoughtful investment selections.  While allowing strongly growing assets to keep doing that, it is prudent not to allow them to become a dangerously large part of your portfolio lest they go off the boil.  So if you selectively trim profitable positions along the way, you can boost your income.