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From this question:

In airliners, shoulder harnesses are not installed for passengers because:

  1. it is not mandated by law
  2. it is too much for average passengers to deal with
  3. it requires money to install
  4. it is superfluous in a crash when compared to the same safety from a lap belt

Flight crew is required to use the shoulder harness (when installed) as it is the law (14 CFR 91.105): [...]

How is 4 true? Why would shoulder harnesses be obligated for flight attendants in their jump seats, but not for passengers?

Against 1 and 2, should shoulder harnesses be installed, at least to enable passengers to decide to use them or not?

Against 3, do the safety benefits not outweigh the costs?

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    $\begingroup$ Look at the force vectors in an airplane crash vs a car crash. Survivable crashes are often vertical with the higher forces where-as a car is more horizontal. The other part is the belt is there to keep you in your seat, not to keep you from impacting the seat in front of you. For a pilot, the shoulder harness is there to keep them from hitting the dash or controls in turbulence, not really for additional crash survivability. $\endgroup$ – Ron Beyer Jun 11 '16 at 23:15
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    $\begingroup$ I think what @ymb1 is trying to say is that if a crash is going to kill people, a shoulder harness isn't going to save them. $\endgroup$ – Ryan Mortensen Jun 11 '16 at 23:51
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    $\begingroup$ On some aircraft the rows that don't have seats in front of them are equipped with airbags in the seatbelt. $\endgroup$ – TomMcW Jun 12 '16 at 1:00
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    $\begingroup$ Going toward cost, the shoulder belt is more complex (therefore more expensive) than the lap belt because, like the shoulder belt in a car, it has to have a mechanism that allows freedom of movement while holding tight when needed. $\endgroup$ – TomMcW Jun 12 '16 at 1:09
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    $\begingroup$ Shoulder harnesses are present on some smaller aircraft. I've been on a Beechcraft King Air with shoulder and lap harnesses. Perhaps answers could address went some manufacturers consider it worth the cost. $\endgroup$ – Cody P Jun 12 '16 at 16:40
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I'll address these one by one.

Should shoulder harnesses be installed, at least to enable passengers to decide to use them or not?

No. They would not serve a significant purpose to improve safety in those seats, and they would add weight and increase maintenance costs for airlines. Keep reading, and this will come to light collectively with my full answer.

Why would shoulder harnesses be obligated for flight attendants in their jump seats, but not for passengers?

Those seats aren't lined up in rows, therefore there is no seat in front of them that they would hit and be stopped by. Think of it like a school bus with no seat belts; the passengers will be stopped by the seat in front of them in a low-force crash, but the bus driver wears a seat belt because there is a steering wheel and windshield in front of them, not a padded seat.

How is 4 true? (It is superfluous in a crash when compared to the same safety from a lap belt)

Essentially, if there is a crash that will kill people, the shoulder harness will not be the thing that makes a difference. The lap belt isn't really even there for crashes. Extreme turbulence could throw someone up out of a seat; a lap belt is sufficient to prevent this.

A shoulder strap is designed to reduce impact with an object on board a vehicle. A car, we can safely assume, will remain in one piece if it hits a concrete wall. Airplanes do not have structures designed to withstand impacts like that. If a plane disintegrates in a crash, the passenger, packaged along with the lap belt, shoulder strap, and all, will come to impact the external object directly due to structural failure. No seat belt can save you from a total structural failure.

Do the safety benefits not outweigh the costs?

Since there really is no safety benefit whatsoever in adding them in the seats that don't already have them, no. It would be security theater that costs airlines a lot of money.

Regarding Pilots

Notice I highlighted the low-force impact statement up above in regards to buses. Let's say there is a low-force impact in a plane such as a taxiing crash that jars everyone on board. The passengers will hit the seats in front of them, not that the seats are even necessary to prevent injury. However the pilots, on the other hand, have a lot of hard instrumentation in front of them such as throttle quadrants, and other things you don't want accidentally shoved forward during a ground crash.

It appears that very little actual research has been done on this subject. For example, this European Transportation Safety Council report states,

Many proposed safety measures require further research before their benefit to cabin safety and the optimal design can be firmly established. In most cases, the availability of accurate, validated analytical models is indispensable.

It also states,

It must be emphasised that these [death count] figures are at best estimates, since insufficient detailed accident information is available.

and,

A fundamental limitation to this process, however, is the lack of adequate accident information for a sufficient number of accidents to allow full cost benefit analyses to be performed. The absence in many accident investigations of detailed information on injury mechanisms and cause of death makes the precise estimation of the potential benefits of any one measure very difficult. The Cherry study (1995)* , one of few studies which have performed a detailed trade-off concerning the full range of possible survivability measures, provides insight into available options. However, the small number of accidents available for investigation in such studies means that the basis for determining priorities still relies heavily upon best expert judgement rather than a truly numerically-specific approach.

...and lastly,

Three-point lap and shoulder harnesses: As mentioned above, passengers who are restrained by a lap belt only and who do not take up a brace position prior to impact, are likely to suffer serious injuries due to the flailing motion of the upper body. As with rearward facing seats, the provision of three-point shoulder harness restraint systems would prevent this situation. If all passenger assumed the brace position prior to impact, the additional benefits of a threepoint shoulder harness would be small. In reality, however, for a variety of reasons, occupants generally do not assume a proper brace position, so a three-point lap and shoulder harness would be likely [to] improve occupant protection substantially.

So, after reading this document, I did not find any numerical evidence contained within to support its claims, and it even states that it has very little research to go on and appears to be written in an almost purely speculative and subjective manner, which leads me back to the original point above...

A shoulder strap will not save you from a structural failure, and if the plane bumps into something while rolling wheels-down on the ground, the seat in front of the passenger will absorb their impact.

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    $\begingroup$ While a shoulder harness is there to protect a pilot in a crash, it's also in place to make sure the pilot stays in place to prevent one in the first place. In extreme circumstances like severe turbulence or excursions from controlled flight the shoulder belt will help save the pilot from injuries which could incapacitate them and prevent them from being able to control the airplane. $\endgroup$ – GdD Jun 13 '16 at 12:27
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    $\begingroup$ Also, during turbulence, the shoulder harness keeps the pilot in contact with the seat so (s)he stays in the same relative position to all the controls, therefore, he is able to continue flying the plane with existing muscle memory instead of having to look for where the control is now that he's half-way into the co-pilot's seat. Obviously, the seat belts in a race car will help protect the driver in many accidents, but the main reason they cinch them up super tight is to remain one with the vehicle so they can remain in control & minimize the chance of a crash in the first place. $\endgroup$ – FreeMan Jun 13 '16 at 20:53
  • $\begingroup$ @GdD and FreeMan thank you guys! Good additions to the information here. +^ $\endgroup$ – Ryan Mortensen Jun 14 '16 at 6:26
  • $\begingroup$ There has been research is survivability of aircraft crashes, and three point seatbelt would most definitely be beneficial in case of a crash. The claims you make in your answer boil down to: seatbelt are only installed for turbulence. Do you have any references that back that up? $\endgroup$ – Koyovis Dec 30 '17 at 3:14
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    $\begingroup$ Shoulder harnesses would provide only marginal increases in safety over lap-belts for passengers, at the cost of considerable complexity to use and weight increases. The trade-off is not warranted. For pilots and flight attendants, that marginal safety incresae is warranted given their roles. That's my guess. $\endgroup$ – Pete855217 Mar 13 at 15:27
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You are asking some very good questions about information that is backed up by research into improvement of survivability rates of airliner crashes, and three-point shoulder harnesses would be beneficial. From this report:

Based on the Cherry study and other information referred to in this report and considering the other aspects mentioned above, three impact protection measures are recommended for priority attention:

  • Improvement of seat-floor strength;
  • Three-point safety harness occupant restraint;
  • Improvement to strength of overhead stowage.

The report cites installation of three-point seat belts to have a similar benefit to mounting the seats backwards, as quantified in Table 1: a reduction of 18% in avoidable impact fatalities for three point seatbelt, 19% for rearward facing seats. Probably within the margin of error of this study.

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The first three of the four bullet points in the OP are factually correct. Installing shoulder harnesses for all passengers has an impact on seat construction, and on dispatch time to make sure that all passengers are wearing them correctly. It's a hassle, and only if shoulder harnesses become mandatory will they be installed everywhere and be able to save lives. An option might be three-point seatbelt like used in cars, since everyone is familiar with them.

OP questions one by one:

How is 4 true? Why would shoulder harnesses be obligated for flight attendants in their jump seats, but not for passengers?

Research shows that 4 is untrue.

Against 1 and 2, should shoulder harnesses be installed, at least to enable passengers to decide to use them or not?

1 and 2 are stated correctly: that is the current situation. Three-point seatbelts or shoulder harnesses that are installed but people don't have to use them would be the worst of both worlds, where the money had been spent but no additional lives are saved. They should be made mandatory to wear once installed, like the 3-point seatbelt in cars. People seem to have a tendency to have to be dragged into using safety belts, an airliner has cabin crew on board who can assist and make sure that everyone is using them.

Against 3, do the safety benefits not outweigh the costs?

Yes they do - but the benefits are distributed amongst different parties. Also, once mandatory, all airlines face these costs in equal amounts and they become invisible for price competition purposes.

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    $\begingroup$ @MichaelKjörling thanks, have edited. Ah yes the downvote, would there be a connection with the rant added to the original answer, I wonder. $\endgroup$ – Koyovis Dec 30 '17 at 21:26

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