Average: 7.3%. The early low returns were due to market stagnation from 1966-1983 when the dow hit a psychological barrier at 1,000 and high interest rates and inflation in the late 70's and early 80's. Recent low returns were from the .com bust in 2000 and the recession in 2008.

Probability of a given minimum return based on the number of years above which exceed it.

Return	<5%	5-7%	7-9%	9-11%	11-14%	All
Years	10	11	7	7	5	40
Probability return ≥ *	≥3%	≥5%	≥7%	≥9%	≥11%
	95%	75%	48%	30%	12%

* You can argue that we know how to control inflation now and we won't have another period like the 70's, but I've worked with economists before and just when they think they've found all the variables that affect a number, something they never thought of comes up.

Why do analysts tell you the average long term gain is 10-12%?
Because they are averaging each yearly gain or loss.
Using Compound Annual Growth Rate (CAGR) - A rate which applied each year over the same period would give the same net gain is more reasonable. E.g.

Value	% gain/loss	Constant 4.83%
$100		$100.00
$120	20%	$104.83
$96	-20%	$109.89
$115	20%	$115.20
Average	6.7%
CAGR	4.83%	Excel function =RATE(3,0,100,-115)

1950-2014

index	Av yearly gain/loss	CAGR
Dow	11.5%	7.0%
S&P	8.8%	7.5%

Note: The averages don't include dividends, which for the Dow have averaged 3-4% over the last 100 years. Recent S&P dividends are closer to 2%, as companies reinvest more of their profits.

See: Bear Market Recoveries Since 1950
See S&P 1950-2014

Return	loss > 20%	loss 10% to 20%	loss 0 to 10%	profit 0-10%	profit 10-20%	Profit 20-40%	profit > 40%
Years	3	9	8	10	18	15	1	64
Probability of return		Loss >10%	Loss	Profit	Profit >10%	Profit >20%
		19%	31%	69%	53%	25%

See also:
Stock Investors Should Expect 6%-7% Annual Return, Buffett Says - Bloomberg, 2014
S&P 1950-
Other Indices
MSCI World Index.
LONG-TERM INVESTMENT PERFORMANCE | Raymond James
Saving

But houses also got larger.

Year	sq ft	price	$/sq ft
1950	1,000	$7,354	$7.35
1970	1,500	$17,002	$11.33
2006	2,500	$227,100	$90.84

Price per square ft increased an average of 4.6%/year from 1950 to 2006.
Note: In metropolitan areas houses can cost more than $200 per sq ft.

Census Bureau data showed:
2006   Median   Average
     $246,500.  $305,900