Decadence is the measure of the overall standard of living for a Muslim dynasty.
Only Muslims with iqta or tribal government use decadence. Muslims living under other government types, such as nomadism or merchant republics, do not use the decadence mechanic.
Effects of Decadence
Depending on your decadence level 
, your realm will receive positive or negative effects. These effects apply to demesne income and troop morale. The standard decadence level is 25%, at which your realm will neither suffer nor gain anything from it. Going below 25% will result in your levies having a higher morale and your Demesne income increasing. Conversely, going above 25% will result in decreased levy morale and decreased demesne income. Both of these effects increase or decrease in correlation with your specific decadence level.
At 60%, you'll get an ambition to reduce decadence. If the Hashashin exists, they may also pay you a visit...
If you go above 75% decadence and are at least a duke, you may become the target of an invasion by righteous tribes who wish to usurp your titles and end your excessive standard of living.
Separately, decadence also lowers your direct vassal limit. Hover the cursor over the limit to see the exact effect.
Decadent characters
Decadence is primarily generated by kinsmen with the 
trait. They contribute monthly decadence until imprisoned, killed, straightened up or banished.
The largest factor affecting whether characters become decadent is rank. Courtiers are the most likely to become decadent, while kings and emperors never do. However, kings and emperors who are decadent before inheriting their titles will continue to be decadent.
A number of traits also affect the chance of becoming decadent:
- Much more likely:
Wicked Caliph (only affects a Caliph, as Muslim realms do not have theocracies), 
, 
, 
- More likely: some unsuccessful educations (
), 
, 
, 
, 
, 
, 
, 
, 
- Less likely: learning education (especially if successful),
, 
- Much less likely:
, 
, 
(only available to Sunni Muslims), 
- Never:
, 
(unavailable before activation of jihads), 
The safest childhood styles for excess family members are probably:
- duty + stewardship: very high chance of diligent or temperate, good as mayors
- faith + learning: learning education boost, chance of zealous, lower intrigue skill if you have to arrest them
Trait events for rulers
Separately, rulers with certain traits can get events that add or remove 2% decadence directly.
Can add 2% decadence:
- , , ,
- All sins:
, , , , 
, , 
- Most misc negative personality traits:
, 
, , , 
, ; 
is seen as unbecoming as well.
Can remove 2% decadence:
, 
, 
, 
, , , 
- All virtues
- Most misc positive personality traits
Limiting dynasty size
Control the number of children being born: Remember that not all wives you marry need to bear children. Having "only" 1 or 2 young wives to bear children is usually enough to secure succession. It also reduces intrigue at home, as wives who don't bear children will have a lesser incentive to plot against the children of other wives. Marrying wives above the age of 40 requires the character finder; marriages arranged via the "rings" button will set the maximum age of brides at 40.
Limit the number of other rulers of your dynasty: Rulers try to marry a number of women based on their rank, and often also seek marriages for male relatives in their court.
Addressing decadent dynasty members
From the Religion tab (F9), you can see the head of your Religion (Sunni/Shi'a Caliph), along with any active Jihads called by your Caliph. Below that is a list of dynasty members who are currently decadent. From here, you can address your decadent dynasty members in a few ways.
Straighten Up Decadent Relatives - Decadent relatives can be "straightened up" via a button in the Religion tab. If they accept, they will immediately lose the Decadent trait and will not become decadent again. If they refuse, you can either gain a reason to imprison them (Takfir); note that invoking Takfir will not remove the Decadent trait. If you have high Diplomacy, Intrigue, or Learning, you can either persuade them further, scare them, or debate them, respectively; if successful, the Decadent trait will also be removed. Each straightening up attempt costs 100 piety. You can also buy a favor from decadent relatives and force them to straighten up using the favor after they refused during the first attempt.
Imprisonment - Another slightly more tyrannical approach is simply to purge the decadent members of your dynasty by granting them an indefinite stay in your dungeon. This is tyranny-free if they refused to straighten up and you invoked Takfir. It may be worthwhile to turn off the automatic ending of known plots, as this may present opportunities for justly imprisoning decadent dynasty members. However, if the character has a high monthly decadence value, this may not be a viable strategy.
Addressing personal decadence
If your own character has the decadent trait, you will not only generate decadence personally, but also find yourself unable to ask other dynasty members to straighten up.
Going on the Hajj is one of the few ways to remove the decadent trait from your own character, assuming nobody can ask you to straighten up. Heads of dynasties with high Learning (>10) have a chance to persuade Decadent relatives to go on the hajj, if the relative is convinced on the second attempt and has yet to go for the hajj.
Become Zealous - The Theology focus has some events which can turn your character Zealous.
Switch to another religion - Just as temporarily becoming Muslim clears kinslayer traits, temporarily becoming non-Muslim clears the decadent trait. If you have 500 piety, Secret religions provide a relatively easy way to switch to another religion and back.
Retire your character - If your heir is a competent adult, they might make a better ruler.
Reducing decadence
Land Grants to Dynasty Members - The easiest and most beneficial way to not only reduce how much decadence per month you receive, but also one of the few ways you can lower your current decadence. Higher tier titles will remove more decadence; patch 2.8 also significantly increased Decadence reduction from title grants. Note that the decadence reduction is scaled according to your dynasty's current decadence. Also, landing dynasty members who had already straightened up is advised.
Holy War - Calling a Holy War and winning will reduce decadence by up to 10%. However, the decadence will only be removed at the end of the war, so plan accordingly. Take care not to lose the war, as losing an offensive holy war can significantly increase decadence.
Jihad - Participating in a Jihad will usually remove large amount of decadence (50-100%) depending on your war contribution. Furthermore, you can ensure your adult sons and other direct subjects gain the trait, which will prevent them from becoming decadent in the future.
Help dynasty members win holy wars: Consider raiding or attacking any realms attacked by your dynasty members in order to ensure they win the war. This is especially important if the attacker is not your vassal, because in that case, a loss could increase dynasty decadence.
Ramadan - Available through your decisions window, Ramadan is a great event chain to lower your decadence and can reduce decadence by 3 points per event. However, if your character has sinful traits, it can backfire as you may get events which force decadence to increase. It also allows you to earn some Piety if successful (15-45 as an average), which greatly helps rulers with low Piety income to earn the Piety required to straighten up relatives. The decision has a cooldown of 5 years and may not be taken if your character has a chronic disease (highly possible if 
Reaper's Due DLC is active).
Furusiayaa Tournament - Available through your decisions window, the Furusiayaa Tournament will lower your decadence by 5 points. Unlike Ramadan, a ruler may only hold the tournament once.
Sieges and Battles - Decadence gained per month will also go down by a small but noticeable amount whenever you or a dynasty member participates in battles or sieges. As such, you should generally make sure either your character or a dynasty member is leading the army. Decadent dynasty members leading armies may also get them killed. Consider adopting a culture that can raid in order to participate in more battles without the risks of declaring war.