diff --git a/src/openvic-simulation/country/CountryInstance.cpp b/src/openvic-simulation/country/CountryInstance.cpp
index 818610c0f..beb8dbdef 100644
--- a/src/openvic-simulation/country/CountryInstance.cpp
+++ b/src/openvic-simulation/country/CountryInstance.cpp
@@ -1841,7 +1841,15 @@ void CountryInstance::update_modifier_sum(Date today, StaticModifierCache const&
 	// TODO - calculate stats for each unit type (locked and unlocked)
 }
 
+void CountryInstance::reserve_for_province_modifier_sum(const std::size_t extra_capacity) {
+	extra_capacity_for_province_modifiers += extra_capacity;
+}
+
 void CountryInstance::contribute_province_modifier_sum(ModifierSum const& province_modifier_sum) {
+	if (extra_capacity_for_province_modifiers > 0) {
+		reserve_more(modifier_sum, extra_capacity_for_province_modifiers);
+		extra_capacity_for_province_modifiers = 0;
+	}
 	modifier_sum.add_modifier_sum(province_modifier_sum);
 }
 
diff --git a/src/openvic-simulation/country/CountryInstance.hpp b/src/openvic-simulation/country/CountryInstance.hpp
index 655e73138..f720d2fde 100644
--- a/src/openvic-simulation/country/CountryInstance.hpp
+++ b/src/openvic-simulation/country/CountryInstance.hpp
@@ -653,8 +653,10 @@ namespace OpenVic {
 
 		bool update_rule_set();
 
+		std::size_t extra_capacity_for_province_modifiers = 0;
 	public:
 		void update_modifier_sum(Date today, StaticModifierCache const& static_modifier_cache);
+		void reserve_for_province_modifier_sum(const std::size_t extra_capacity);
 		void contribute_province_modifier_sum(ModifierSum const& province_modifier_sum);
 		fixed_point_t get_modifier_effect_value(ModifierEffect const& effect) const;
 		constexpr void for_each_contributing_modifier(
diff --git a/src/openvic-simulation/map/MapInstance.cpp b/src/openvic-simulation/map/MapInstance.cpp
index 4b864e85b..bb126c24d 100644
--- a/src/openvic-simulation/map/MapInstance.cpp
+++ b/src/openvic-simulation/map/MapInstance.cpp
@@ -143,6 +143,10 @@ void MapInstance::update_modifier_sums(const Date today, StaticModifierCache con
 	for (ProvinceInstance& province : get_province_instances()) {
 		province.update_modifier_sum(today, static_modifier_cache);
 	}
+
+	for (ProvinceInstance& province : get_province_instances()) {
+		province.update_country_modifier_sum();
+	}
 }
 
 void MapInstance::update_gamestate(InstanceManager const& instance_manager) {
diff --git a/src/openvic-simulation/map/ProvinceInstance.cpp b/src/openvic-simulation/map/ProvinceInstance.cpp
index e7bc0bd60..dce65cb4c 100644
--- a/src/openvic-simulation/map/ProvinceInstance.cpp
+++ b/src/openvic-simulation/map/ProvinceInstance.cpp
@@ -288,6 +288,12 @@ void ProvinceInstance::update_modifier_sum(Date today, StaticModifierCache const
 		modifier_sum.add_modifier(*crime);
 	}
 
+	if (controller != nullptr) {
+		controller->reserve_for_province_modifier_sum(modifier_sum.get_modifiers().size());
+	}
+}
+
+void ProvinceInstance::update_country_modifier_sum() {
 	if (controller != nullptr) {
 		controller->contribute_province_modifier_sum(modifier_sum);
 	}
diff --git a/src/openvic-simulation/map/ProvinceInstance.hpp b/src/openvic-simulation/map/ProvinceInstance.hpp
index f280155c7..b3ffc3dfa 100644
--- a/src/openvic-simulation/map/ProvinceInstance.hpp
+++ b/src/openvic-simulation/map/ProvinceInstance.hpp
@@ -184,6 +184,7 @@ namespace OpenVic {
 		size_t get_pop_count() const;
 
 		void update_modifier_sum(Date today, StaticModifierCache const& static_modifier_cache);
+		void update_country_modifier_sum();
 		fixed_point_t get_modifier_effect_value(ModifierEffect const& effect) const;
 
 		void for_each_contributing_modifier(ModifierEffect const& effect, ContributingModifierCallback auto callback) const {
diff --git a/src/openvic-simulation/modifier/ModifierSum.cpp b/src/openvic-simulation/modifier/ModifierSum.cpp
index 1c4daba47..9beba54eb 100644
--- a/src/openvic-simulation/modifier/ModifierSum.cpp
+++ b/src/openvic-simulation/modifier/ModifierSum.cpp
@@ -1,10 +1,11 @@
 #include "ModifierSum.hpp"
 
-#include "openvic-simulation/modifier/Modifier.hpp"
+#include <variant>
 
-#include "openvic-simulation/country/CountryInstance.hpp"
-#include "openvic-simulation/map/ProvinceInstance.hpp"
 #include "openvic-simulation/core/template/Concepts.hpp"
+#include "openvic-simulation/country/CountryInstance.hpp" // IWYU pragma: keep for modifier_source_t
+#include "openvic-simulation/map/ProvinceInstance.hpp" // IWYU pragma: keep for modifier_source_t
+#include "openvic-simulation/modifier/Modifier.hpp"
 
 using namespace OpenVic;
 
@@ -30,10 +31,6 @@ void ModifierSum::clear() {
 	value_sum.clear();
 }
 
-bool ModifierSum::empty() {
-	return modifiers.empty();
-}
-
 fixed_point_t ModifierSum::get_modifier_effect_value(ModifierEffect const& effect, bool* effect_found) const {
 	return value_sum.get_effect(effect, effect_found);
 }
@@ -60,8 +57,6 @@ void ModifierSum::add_modifier(
 }
 
 void ModifierSum::add_modifier_sum(ModifierSum const& modifier_sum) {
-	reserve_more(modifiers, modifier_sum.modifiers.size());
-
 	// We could test that excluded_targets != ALL_TARGETS, but in practice it's always
 	// called with an explcit/hardcoded value and so won't ever exclude everything.
 	for (modifier_entry_t const& modifier_entry : modifier_sum.modifiers) {
diff --git a/src/openvic-simulation/modifier/ModifierSum.hpp b/src/openvic-simulation/modifier/ModifierSum.hpp
index 5e5c78810..56d680e7a 100644
--- a/src/openvic-simulation/modifier/ModifierSum.hpp
+++ b/src/openvic-simulation/modifier/ModifierSum.hpp
@@ -105,8 +105,20 @@ namespace OpenVic {
 		ModifierSum& operator=(ModifierSum const&) = default;
 		ModifierSum& operator=(ModifierSum&&) = default;
 
+		constexpr std::size_t capacity() const {
+			return modifiers.capacity();
+		}
+		constexpr std::size_t size() const {
+			return modifiers.size();
+		}
+		constexpr bool empty() const {
+			return modifiers.empty();
+		}
+
+		constexpr void reserve(const std::size_t new_capacity) {
+			modifiers.reserve(new_capacity);
+		}
 		void clear();
-		bool empty();
 
 		fixed_point_t get_modifier_effect_value(ModifierEffect const& effect, bool* effect_found = nullptr) const;
 		bool has_modifier_effect(ModifierEffect const& effect) const;
@@ -117,7 +129,7 @@ namespace OpenVic {
 			modifier_entry_t::modifier_source_t const& source = {},
 			ModifierEffect::target_t excluded_targets = ModifierEffect::target_t::NO_TARGETS
 		);
-		// Reserves space for the number of modifier entries in the given sum and adds each of them using add_modifier
+		// Loops through modifiers calling add_modifier
 		// with the modifier entries' attributes as arguments. This means non-null sources are preserved (null ones are
 		// replaced with this_source, but in practice the other sum should've set them itself already) and exclusion targets
 		// are combined with this_excluded_targets.