Idea groups

For the ideas of individual countries, see national ideas.

For the ideas of American natives, see native ideas.

For the practical guide on idea groups, with analysis and recommendations, see Idea groups guide.

As a nation develops technologically, it unlocks the ability to gain additional idea groups. Investing into a full idea group can take a long time and cost several decades worth of power. The act of choosing at any point of time a certain idea group over the other and advancing in it represent the time and focus nations choose to spend historically along certain administrative, diplomatic, and military specializations.

Mechanics summary

Each nation can have up to 8 idea groups from a pool of 19 groups (only 18 are shown as the Aristocratic and Plutocratic idea groups are locked behind exclusive government reforms). Idea group choice is only restricted by the requirement for a balanced mix of groups: No more than half of a nation's chosen idea groups, whether completed or not, can have the same associated type of monarch power.^[1] This limit can be turned off in options before starting a game without disabling achievements.

Idea group choices are unrestricted, with several exceptions: Plutocratic ideas are enabled for most republics but disabled for most other government types; Aristocratic ideas are disabled for most republics but enabled for most other government types; Divine ideas are enabled only for theocracies; Horde Government ideas are enabled only for steppe nomads; Indigenous ideas are enabled only for native tribes and Maritime and Naval ideas are disabled for primitives. However, AI nations have weights assigned to the idea groups to make their choices random but still somewhat historical.

Each idea group consists of 7 ideas. Advancing within an idea group is done sequentially and each unlocked idea enhances a particular aspect of the nation. Unlocking the last idea in the group also unlocks the group's bonus.

Unlocking new slots

New idea group slots are unlocked with the advancement in Administrative technology as described below:

Administrative tech level	5	7	10	14	18	22	26	29
Number of idea groups	1	2	3	4	5	6	7	8
Year in which the technology loses its ahead of time penalty	1453	1479	1518	1570	1622	1674	1730	1775

Abandoning idea groups

Abandoning an idea group can be used to free up an idea slot if it no longer serves the nation and there may be others more suitable groups that do so. It is important to note, however, that doing so will result in losing all the given bonuses from said idea group and will only refund 10% of the monarch power points invested in the group^[2]. Discarding idea groups will also impact the number of Ideas a nation has towards unlocking National Ideas and Ambitions.

Due to the low refund rate and the importance of monarch power points, picking an idea group should be done with its future usefulness to the nation in mind. However, this option should not be disregarded either as it is extremely useful in the right circumstances. For example:

Primitives can take only the first 3-4 Exploration ideas, get a colonial border with a western power, then drop Exploration for something else. Some would benefit more from the Expansion idea group; but Expansion doesn't allow Explorers or long-range island-hopping. They could even opt out of colonization altogether after getting the border they needed.

Technology cost reduction

Each unlocked idea grants a −2% Technology cost reduction. The reduction can result in up to −14% for a fully unlocked idea group (without accounting for further reductions that may be applied from the ideas themselves). The type of the reduction depends on the affiliation of the idea group; for example, the Diplomatic idea group grants Diplomatic tech cost reduction.

Innovativeness gain

Being the first nation to unlock an idea or unlocking an idea within 365 days after it is first unlocked gives a nation a base of +2 innovativeness.

Policies

Main article: Policies

Policies become enabled when a pair of 2 idea groups are fully unlocked. Picking a policy provides a bonus to the nation at the monthly cost of 1 monarch power per policy (type of monarch power used depends on policy affiliation) if the limit is overwhelmed (base: 1 policy of each type can be picked for free) . Once picked, a policy cannot be canceled for a duration of 10 years. There is no restriction on policy composition and a nation can activate up to 9 policies at a time, 3 per type.

Idea cost

Unlocking an idea has a base idea cost of 400 monarch power^[3] (the type of power depending on the idea group).

The base idea cost is modified by the following:

	Conditions
−25%	for the AI at very hard difficulty
−1%	per −1% modifier to all power costs (up to −10% with 100 Innovativeness and another −10% during a Golden Era)
−10%	as Tengri with Protestant or Anglican as secondary faith as Shinto with “Selective Integration” isolationism level as Confucian country harmonized with Mahayana
−5%	with a ruler which has the ‘Free Thinker’ personality with ‘Mughal Diwan’ government with assimilation bonus for the Korean culture as Mahayana country as Protestant country with “Individual creeds” aspect
+0.2%	as Nahuatl countries for each point of doom
+20%	during the Dominance of the Clergy disaster during the Dominance of the Brahmins disaster
+25%	during the Janissary Decadence disaster
+50%	if bankrupt during the Dominance of the Tribes disaster

Ideas and policies:

	Traditions	Ideas	Bonuses	Policies
−10%	Aztec traditions Bregenzer traditions Farsi traditions Great Ming traditions Incan traditions Rostov traditions Sardinian traditions Sardinian-Piedmontese traditions Urbinate traditions	Indigenous idea 6: Trade with Foreigners American Southwest idea 1: The Old Tales Andalusian idea 4: Legacy of Ibn Arabi Antemoro idea 2: Sorabe Aymaran idea 1: Legacy of Tiwanaku Bahmani idea 7: Great Madrasa of the Deccan Bengal Sultanate idea 4: Attract Sufi Scholars Brunswicker idea 4: Herzog August Library Central Algonquian idea 4: Teaching Scrolls Cherokee idea 4: Cherokee Adaptability Cilli idea 4: Slovenian Reformation Colonial idea 4: Free Thinkers Daimyo idea 2: The Bun and the Bu Date idea 3: Renga Group Deccani Sultanate idea 2: Promote Deccani Regional Identity Epirote idea 4: Metropolis of Knowledge Federation idea 4: Native Sign Language Greek idea 7: Greek Enlightenment Hamburger idea 5: School of the Johanneum Hanseatic idea 6: Spread the Northern Style Hessian idea 2: Marburg University Hojo idea 3: Reform the Administration Imagawa idea 3: History of the Imagawa Korean idea 7: The Kyujanggak Leonese idea 4: School of Salamanca Lotharingian idea 4: New Carolingian Renaissance Madyas idea 1: Hinilawod Mainzian idea 2: Center of Religious Learning Malaccan idea 7: Jawi Script Mantuan idea 2: The Countess's Studiolo Maravi idea 1: Fresh Perspectives Mesoamerican idea 1: Olmec Legacy Mexican idea 4: Casa de las Campanas Milanese idea 1: Renaissance Prince Moroccan idea 2: Sufi Marabouts Mughal idea 3: Ibadat Khana - House of Worship Native idea 2: Storytelling Tradition Northumbrian idea 6: The Literary and Philosophical Society Orleanaise idea 6: Calvin and Molière PLC idea 2: Jagiellonian University Pisan idea 3: The University of Pisa Polotskian idea 3: Cultural Center Savoyard idea 5: Savoyard Flexibility Shawnee idea 4: Shaawanwaki Identity Siamese idea 7: Education Reforms Takeda idea 5: Koshu Hatto Thüringian idea 6: Weimar Classicism Tibetan idea 7: The Pale Earth School Toki idea 6: Innovation Over Tradition Trebizond idea 3: Legacy of the Alexiad Tsutsui idea 7: Tradition and Innovation Uesugi idea 7: Denoku no Ji Welsh idea 6: Prifysgolion Cymru Westphalian idea 1: Jewish Rights	Full Humanist Clevian ambition Granadan ambition Samoan ambition Transoxianian ambition	Humanist-Trade: Exchange of Ideas
−5%	—	Frankfurter idea 1: Establish the Buchmesse Italian (minor) idea 1: Italian Renaissance Kilwan idea 4: Cultural Melting Pot Malayan Sultanate idea 4: Classical Malay Swahili idea 4: Indian Ocean Melting Pot Tuscan idea 1: Birthplace of the Renaissance	—	—

Several missions, decisions and events grant modifiers that affect idea cost.

National ideas

Main article: National ideas

Every country has seven national ideas. These ideas do not cost any monarch points but instead are unlocked; one national idea is unlocked for every third idea bought normally from an idea group.

Many nations have unique national ideas while a large number of others have national ideas based upon a common geographic region or a shared culture. The rest of the nations, who lack unique NIs or don't share a common NI group, draw their national ideas from a common generic set of ideas.

National tradition: Every nation also starts with a national tradition: two abilities which define the history and heritage of the country.

National ambition: Countries also have a “national ambition”, which is a bonus that is unlocked when a nation has gained all seven of its national ideas. This bonus is also unique for each country.

Administrative groups

Administrative power is required to unlock ideas within these groups.

Administrative

Organized Mercenary Payments

−10% Mercenary cost

Adaptability

−25% Core-creation cost

Benefits for Mercenaries

−15% Mercenary maintenance

Bookkeeping

−0.5 Interest per annum

Mercenary Recruitment

+25% Mercenary manpower

Administrative Efficiency

+1 Possible advisor

Civil Service

−10% Administrative technology cost

Bonus:

+25% Governing capacity modifier

Associated events: Administrative idea group events

Economic

Bureaucracy

+10% National tax modifier

Organized Construction

−10% Construction cost

National Bank

+0.10 Yearly inflation reduction

Debt and Loans

−0.5 Interest per annum

Centralization

−0.05 Monthly autonomy change

Nationalistic Enthusiasm

−5% Land maintenance modifier

Smithian Economics

+10% Production efficiency

Bonus:

−10% Development cost

Associated events: Economic idea group events

Expansion

Additional Colonists

+1 Colonist

Additional Merchants

+1 Merchant

Faster Colonists

+20 Global settler increase

Factories

−50% Fort maintenance on border with rival

−20% Center of trade upgrade cost

Additional Diplomats

+1 Diplomatic relation

General Colonization Law

+1 Colonist

+5% Settler chance

Competitive Merchants

+20% Global trade power

Bonus:

−10% Minimum autonomy in territories

Associated events: Expansion idea group events

Humanist

Tolerance

+25% Religious unity

Local Traditions

−2 National unrest

+25% Religious harmonization speed

Ecumenism

+2 Tolerance of heretics

Indirect Rule

−10 Years of separatism

Cultural Ties

+2 Max promoted cultures

Benevolence

+30% Improve relations

Humanist Tolerance

+2 Tolerance of heathens

Bonus:

−10% Idea cost

+0.25 Yearly harmony increase

+10% Brahmins loyalty equilibrium for muslims

+10% Dhimmi loyalty equilibrium

Associated events: Humanist idea group events

Innovative

Patron of the Arts

−1% Prestige decay

Empiricism

+50% Innovativeness gain

Scientific Revolution

−10% Technology cost

−10% Institution embracement cost

Dynamic Court

+1 Possible advisor

Print Culture

+25% Institution spread

Optimism

−0.05 Monthly war exhaustion

+1 Leader without upkeep

Expanded Policies

+1 Free policies

Bonus:

−25% Advisor costs

Associated events: Innovative idea group events

Religious

Missionary Schools

+1 Missionary

Church Attendance Duty

−25% Stability cost modifier

Divine Supremacy

+3% Missionary strength

Devoutness

+2 Tolerance of the true faith

+2 Yearly papal influence

+0.5 Yearly devotion

+0.25 Monthly fervor

+10% Church power

+0.1 Monthly piety accelerator

+10% Yearly karma decay

+1 Yearly doom reduction

+0.15 Yearly authority

+0.5% Yearly patriarch authority

Religious Tradition

+1 Yearly prestige

Inquisition

+0.1 Prestige per development from Missionary

−50% Missionary maintenance cost

Deus Vult

Permanent casus belli against neighboring heathens and heretics

Bonus:

−25% Culture conversion cost

+10% Brahmins loyalty equilibrium

+10% Clergy loyalty equilibrium

Associated events: Religious idea group events

Diplomatic groups

Diplomatic power is required to unlock ideas within these groups.

Diplomatic

Foreign Embassies

+1 Diplomat

Cabinet

+1 Diplomatic relation

Grand Banquets

+1 Diplomat

Benign Diplomats

+25% Improve relations

Experienced Diplomats

+2 Diplomatic reputation

Flexible Negotiations

−20% Province warscore cost

Diplomatic Corps

−10% Diplomatic technology cost

Bonus:

Lowered impact on stability from diplomatic actions

Associated events: Diplomatic idea group events

Espionage

Claim Fabrication

May fabricate claims for subjects

−25% Cost to fabricate claims

State Propaganda

−20% Aggressive expansion impact

Efficient Spies

+50% Spy network construction

+10% Siege ability

Agent Training

+1 Diplomat

−15% Advisor cost

Vetting

+33% Foreign spy detection

-0.2 Yearly corruption

Privateers

+33% Privateer efficiency

+25% Chance to capture enemy ships

Blackmailing

+15 Vassalization acceptance

+33% Monthly favor gain modifier

Bonus:

+100% Rebel support efficiency

-100% Covert action relations impact

Associated events: Espionage idea group events

Exploration

Quest for the New World

Allows recruitment of explorers & conquistadors

Colonial Ventures

+1 Colonist

Overseas Exploration

+50% Colonial range

Land of Opportunity

+10 Global settler increase

Viceroys

+10% Global tariffs

−20% Envoy travel time

Free Colonies

+10% Settler chance

Global Empire

+25% Naval force limit modifier

+25% Treasure fleet income

Bonus:

Can fabricate claim overseas in colonial regions

Associated events: Exploration idea group events

Influence

Tribute System

+25% Income from vassals

Additional Loyalist Recruitment

−15% Liberty desire in subjects

Integrated Elites

−25% Diplomatic annexation cost

Buffer States

+1 Diplomatic relation

Diplomatic Influence

+2 Diplomatic reputation

Postal Service

−25% Envoy travel time

Marcher Lords

+100% Vassal force limit contribution

Bonus:

−50% Unjustified demands

Associated events: Influence idea group events

Maritime

Notes:

Not available for primitives.

Merchant Traditions

+25% Trade range

+100% Naval tradition from protecting trade

Merchant Marine

+50% National sailors modifier

+25% Marines force limit

Sheltered Ports

+10% Global ship repair

−10% Sailor maintenance

Grand Navy

+50% Naval force limit modifier

+25% Ship tradepower propagation

Ship's Penny

−10% Ship costs

Seahawks

+1 Leader without upkeep

−25% Admiral cost

Naval Fighting Instruction

+50% Blockade efficiency

+25% Privateer efficiency

Bonus:

Ships can repair when in coastal sea zones

+1 Fleet movement speed

Associated events: Maritime idea group events

Trade

Shrewd Commerce Practice

+20% Global trade power

Free Trade

+1 Merchant

Merchant Adventures

+25% Trade range

National Trade Policy

+10% Trade efficiency

-25% Cost to promote Mercantilism

Overseas Merchants

+1 Merchant

Trade Manipulation

+25% Trade steering

Fast Negotiations

+25% Caravan power

Bonus:

+1 Merchant

+10% Burghers loyalty equilibrium

+10% Jains loyalty equilibrium

+10% Vaisyas loyalty equilibrium

Associated events: Trade idea group events

For Exploration note the hidden bonuses when using explorers: each terra incognita sea zone revealed through naval exploration grants +1 naval tradition.

Military groups

Military power is required to unlock ideas within these groups.

Government-based idea groups

Aristocratic

Notes:

Enabled by most monarchies (except the Plutocracy and Feudal Theocracy reform), tribes (except steppe nomads and Great Mongol State), and the Noble Elite republican reform.

Noble Knights

−20% Cavalry cost

+15% Cavalry combat ability

Military Traditions

−10% Military technology cost

Local Nobility

−0.025 Monthly autonomy change

+0.5 Yearly absolutism

Serfdom

−5% Development cost

+15% National manpower modifier

Noble Officers

−1% Yearly army tradition decay

−1% Yearly navy tradition decay

International Diplomacy

+1 Diplomat

−10% General cost

Noble Connections

+25% Mercenary manpower

Bonus:

+1 Leader siege

+10% Nobility loyalty equilibrium

Associated events: Aristocratic idea group events

Divine

Notes:

Available for theocracies and countries with the Feudal Theocracy reform

Servants of God

−5% Development cost

+1 Yearly devotion

+0.5 Yearly legitimacy

By the Grace of God

−10% Fire damage received

Friends in High Places

−33% Cost of reducing war exhaustion

−10% Leader cost

Flesh Is Weak

+10% Morale of armies

Alpha and Omega

−10% Culture conversion cost

−10% Cost of enforcing religion through war

Conviction of Sin

−2 National unrest

Martyrs

+20% Manpower in true faith provinces

Bonus:

+1% Missionary strength vs heretics

+10% Clergy loyalty equilibrium

+10% Brahmins loyalty equilibrium

Associated events: Divine idea group events

Horde Government

Notes:

Enabled only for countries with the Steppe Nomads or Great Mongol State government reforms

Horse-lords of the Steppes

−33% Cavalry cost

+10% Cavalry to infantry ratio

Beyond the Sun

−10% Aggressive expansion impact

Mandate of the Khan

+25% Religious unity

There Shall Be No Grass

−20% Land attrition

Horde Loyalty

−1 National unrest

Watchers of the Silk Road

+20% Caravan power

A Magnanimous Empire

+2 Max promoted cultures

Bonus:

+25% Cavalry combat ability

+10% Tribes loyalty equilibrium

Associated events: Horde Government idea group events

Indigenous

Notes:

Only available for native tribes

Bountiful Land

−5% Development cost

+0.015 Tribal development growth

Irregular Warfare

+1 Attrition for enemies

The Great Law

+25% Monthly reform progress modifier

Braves

+10% Morale of armies

Controlled Burns

+15% National manpower modifier

Trade with Foreigners

−10% Idea cost

Treaties Kept

−20% Province warscore cost

Bonus:

+25% Institution spread

Associated events: Indigenous idea group events

Plutocratic

Notes:

Enabled by most republics (except the Noble Elite reform) and monarchies with the Plutocracy reform.

Tradition of Payment

−20% Mercenary cost

+5% Mercenary discipline

Abolished Serfdom

+10% Morale of armies

Bill of Rights

−2 National unrest

Free Merchants

+1 Merchant

Free Subjects

+10% Goods produced modifier

Free Cities

−5% Development cost

+25% Caravan power

Emancipation

+20% Manpower recovery speed

Bonus:

+10% Institution spread

+10% Burghers loyalty equilibrium

+10% Vaishyas loyalty equilibrium

Associated events: Plutocratic idea group events

General military idea groups

Defensive

Battlefield Commissions

+100% Army tradition from battles

Military Drill

+15% Morale of armies

Improved Maneuver

+1 Land leader maneuver

Regimental System

−5% Land maintenance modifier

Defensive Mentality

−10% Fort maintenance

+20% Fort defense

Supply Trains

+33% Reinforce speed

Improved Foraging

−25% Land attrition

Bonus:

+1 Attrition for enemies

+1 Max attrition for enemies

Associated events: Defensive idea group events

Naval

Notes:

Not available for primitives.

Boarding Parties

+1 Naval leader shock

Improved Rams

+25% Galley combat ability

Naval Cadets

+1 Naval leader fire

−33% Morale hit when losing a ship

Naval Glory

+1 Yearly navy tradition

+1 Blockade impact on siege

Press Gangs

+25% Sailor recovery speed

+25% Marines force limit

Oak Forests for Ships

+20% Heavy ship combat ability

+10% Ship durability

Superior Seamanship

+10% Morale of navies

+10% Global naval engagement

Bonus:

-100% Naval barrage cost

+15% Ship disengagement chance

Associated events: Naval idea group events

Offensive

Bayonet Leaders

+1 Land leader shock

National Conscripts

−10% Recruitment time

Superior Firepower

+1 Land leader fire

Glorious Arms

+100% Prestige from land battles

Engineer Corps

+20% Siege ability

Grand Army

+15% Land force limit modifier

+10% Special unit force limit

Esprit de Corps

+5% Discipline

Bonus:

+5% Recover army morale speed

Associated events: Offensive idea group events

Quality

Private to Marshal

+10% Infantry combat ability

Quality Education

+0.5 Yearly army tradition

+0.5 Yearly navy tradition

Finest of Horses

+10% Cavalry combat ability

Corvettes

+5% Ship durability

Naval Drill

+10% Morale of navies

Copper Bottoms

−25% Naval attrition

Massed Battery

+10% Artillery combat ability

Bonus:

+5% Discipline

Associated events: Quality idea group events

Quantity

Levée en Masse

+25% National manpower modifier

The Young can Serve

+20% Manpower recovery speed

Enforced Service

−10% Regiment cost

Camp Followers

+33% National supply limit modifier

Conscripted Garrisons

+25% Garrison size

The Old and Infirm

−5% Land maintenance modifier

Expanded Supply Trains

−10% Land attrition

Bonus:

+25% Land force limit modifier

Associated events: Quantity idea group events

Events

Main article: List of idea group events

Idea group events are pulse based events. They occur at regular intervals (5 years) based on the nation's idea groups (not all groups are on the same interval).

In addition, the choice of ideas also has some influence in how often an event may happen, known as MTTH; meaning they can increase or decrease the likelihood of a certain to happened in over other events in their pulse group.

AI preference

The AI is dynamically determined according to the current conditions of the country.^[4] To decide which idea group it should take, the AI employs a "weight system": each idea group is given a certain weight and the higher this weight is for an idea group, the higher the chances that the AI will choose it. The AI will never choose an idea group with a weight of 0.^[5]

Idea group	Base weight	Weight modifiers^[6]
Aristocratic	0.45	×0.5 is a subject nation
Plutocratic	0.4
Horde Government	20
Divine	0.9	×0 has the Humanist idea group
Indigenous	1
Innovative	0.3	×2 has enacted “Steppe Nomads” government reform x2 has at least 20 provinces x2 has at least 50 provinces
Religious	0.9	×0 has the Humanist idea group
Espionage	0.75
Diplomatic	0.67	×2 is the emperor of the Holy Roman Empire ×1.5 has at least 2 vassals
Offensive	0.94
Defensive	0.77
Trade	0.7	×0 does not use “Merchant Republic” mechanics and has fewer than 6 cities x3 uses “Merchant Republic” mechanics
Economic	0.85	×1.5 has at least 5 inflation ×1.5 has at least 10 inflation ×2 has enacted “Steppe Nomads” government reform x1.3 has at least 20 cities x1.3 has at least 50 cities
Exploration	0.75	×0 is a colonial nation ×0 is tribal ×0 is a subject nation ×0 is primitive ×0 If it is currently the “Age of Revolutions” ×0 If: does not own a coastal province that is not bordering the Eastern Mediterranean sea zone does not own a province that: is connected to the capital has an empty neighbor province. ×0.5 If: does not own a coastal province that: is not bordering the Mediterranean (without Straits of Gibraltar and Gulf of Almeria) Red Sea or Baltic Sea sea zone does not own a province that: is connected to the capital has an empty neighbor province. ×0.1 If: has less than 3 home ports does not own a province that: is connected to the capital has an empty neighbor province. ×0.1 if does not have the Renaissance institution is not Portugal is not Castile ×0.1 if the capital province is not in France (region), Iberia, Britain or the Low Countries is not in the High American technology group ×0.5 is Aragon ×0.1 it is before 1490 and the capital is not in Iberia ×10 is in the High American technology group ×2 has at least 5 home ports ×2 has at least 10 home ports ×2 has at least 15 home ports ×2 has at least 20 home ports ×1000 is Portugal, Castile or Spain ×2 is Great Britain, the Netherlands, England, France or Kurland
Maritime	0.41	×0 is tribal ×0 is primitive ×0 has no home ports and is not a colonial nation ×0.41 has less than 10 home ports ×0.41 has at least 50 cities ×10 has enacted “Pirate Republic” government reform
Quality	0.49
Quantity	1
Expansion	0.48	×10 has exploration idea group or is a colonial nation x5 is Castile, Portugal or Spain x10 if the Third Rome DLC is not enabled is Russia has at least 20 cities borders an uncolonized province ×0 if does not have exploration idea group is not a colonial nation does not border an empty province ×0.1 is not in one of the following technology groups: Western, Eastern, Muslim.
Administrative	0.96	×2 has enacted “Steppe Nomads” government reform
Humanist	0.9	×0 has the Religious idea group
Influence	0.5	×1.5 is the emperor of the Holy Roman Empire ×1.7 has at least 1 vassal ×1.7 has at least 2 vassals
Naval	0.31	×0 is tribal or primitive ×0 has no home ports and is not a colonial nation ×0.31 has less than 10 home ports ×0.31 has at least 50 cities ×10 has enacted “Pirate Republic” government reform

Footnotes

↑ See in /Europa Universalis IV/common/defines.lua: MAX_IDEA_GROUPS_FROM_SAME_CATEGORY = 0.5,
↑ See in /Europa Universalis IV/common/defines.lua: ABANDON_IDEAGROUP_REFUND = 0.10, -- The part of the idea group spent that will be refunded upon abandonment.
↑ See in /Europa Universalis IV/common/defines.lua: PS_BUY_IDEA = 400
↑ It is still possible to revert to the old system by changing "AI_USES_HISTORICAL_IDEA_GROUPS" to 1 in /Europa Universalis IV/common/defines.lua. Note, however, that the AI will never use historical idea groups in a custom/random setup.
↑ The weights and their modifiers are defined in /Europa Universalis IV/common/ideas/00_basic_ideas.txt at the bottom of each idea group (ai_will_do).
↑ Multiple valid modifiers will stack