diff --git a/a_sync/decorator.py b/a_sync/decorator.py
index 80b5f55f..6b505482 100644
--- a/a_sync/decorator.py
+++ b/a_sync/decorator.py
@@ -1,6 +1,6 @@
 
 import functools
-from typing import Callable, TypeVar
+from typing import Callable, TypeVar, Literal, Optional
 
 from typing_extensions import ParamSpec  # type: ignore
 
@@ -10,25 +10,36 @@
 T = TypeVar("T")
 
 
-def a_sync(coro_fn: Callable[P, T]) -> Callable[P, T]:  # type: ignore
+def a_sync(default: Optional[Literal['sync','async']] = None) -> Callable[[Callable[P, T]], Callable[P, T]]:
     f"""
     A coroutine function decorated with this decorator can be called as a sync function by passing a boolean value for any of these kwargs: {_helpers._flag_name_options}
     """
+    if default not in ['async', 'sync', None]:
+        if callable(default):
+            return a_sync()(default)
+        raise ValueError(f"'default' must be either 'sync', 'async', or None. You passed {default}.")
     
-    _helpers._validate_wrapped_fn(coro_fn)
+    def a_sync_deco(coro_fn: Callable[P, T]) -> Callable[P, T]:  # type: ignore
+    
+        _helpers._validate_wrapped_fn(coro_fn)
 
-    @functools.wraps(coro_fn)
-    def a_sync_wrap(*args: P.args, **kwargs: P.kwargs) -> T:  # type: ignore
-        # If a flag was specified in the kwargs, we will defer to it.
-        for flag in _helpers._flag_name_options:
-            if flag in kwargs:
-                val = kwargs.pop(flag)
-                if not isinstance(val, bool):
-                    raise TypeError(f"'{flag}' must be boolean. You passed {val}.")
-                return _helpers._await_if_sync(  # type: ignore
-                    coro_fn(*args, **kwargs),
-                    val if flag == 'sync' else not val
-                )
-        # No flag specified in the kwargs, we will just return the awaitable.
-        return coro_fn(*args, **kwargs)
-    return a_sync_wrap
+        @functools.wraps(coro_fn)
+        def a_sync_wrap(*args: P.args, **kwargs: P.kwargs) -> T:  # type: ignore
+            # If a flag was specified in the kwargs, we will defer to it.
+            for flag in _helpers._flag_name_options:
+                if flag in kwargs:
+                    val = kwargs.pop(flag)
+                    if not isinstance(val, bool):
+                        raise TypeError(f"'{flag}' must be boolean. You passed {val}.")
+                    return _helpers._await_if_sync(
+                        coro_fn(*args, **kwargs),
+                        val if flag == 'sync' else not val
+                    )
+                
+            # No flag specified in the kwargs, we will defer to 'default'.
+            return _helpers._await_if_sync(
+                coro_fn(*args, **kwargs),
+                True if default == 'sync' else False
+            )
+        return a_sync_wrap
+    return a_sync_deco
diff --git a/tests/test_decorator.py b/tests/test_decorator.py
index 76984946..944ce5bb 100644
--- a/tests/test_decorator.py
+++ b/tests/test_decorator.py
@@ -1,13 +1,86 @@
 
 import asyncio
+import pytest
+from typing import Literal
 
 import a_sync
 
-@a_sync.a_sync
-async def some_test_fn() -> int:
-    return 2
 
-def test_decorator():
+def _test_kwargs(fn, default: Literal['sync','async',None]):
+    # force async
+    assert asyncio.get_event_loop().run_until_complete(fn(sync=False)) == 2
+    assert asyncio.get_event_loop().run_until_complete(fn(asynchronous=True)) == 2
+    # force sync
+    with pytest.raises(TypeError):
+        assert asyncio.get_event_loop().run_until_complete(fn(sync=True)) == 2
+    with pytest.raises(TypeError):
+        assert asyncio.get_event_loop().run_until_complete(fn(asynchronous=False)) == 2
+    assert fn(sync=True) == 2
+    assert fn(asynchronous=False) == 2
+    if default == 'sync':
+        assert fn() == 2
+    elif default == 'async':
+        assert asyncio.get_event_loop().run_until_complete(fn()) == 2
+    elif default is None:
+        assert asyncio.get_event_loop().run_until_complete(fn()) == 2
+    else:
+        raise NotImplementedError(default)
+
+def test_decorator_no_args():
+    @a_sync.a_sync
+    async def some_test_fn() -> int:
+        return 2
+    assert asyncio.get_event_loop().run_until_complete(some_test_fn()) == 2
+    _test_kwargs(some_test_fn, None)
+    
+    @a_sync.a_sync()
+    async def some_test_fn() -> int:
+        return 2
+    assert asyncio.get_event_loop().run_until_complete(some_test_fn()) == 2
+    _test_kwargs(some_test_fn, None)
+    
+def test_decorator_default_none_arg():
+    @a_sync.a_sync(None)
+    async def some_test_fn() -> int:
+        return 2
+    asyncio.get_event_loop().run_until_complete(some_test_fn()) == 2
+    _test_kwargs(some_test_fn, None)
+    
+def test_decorator_default_none_kwarg():
+    @a_sync.a_sync(default=None)
+    async def some_test_fn() -> int:
+        return 2
+    asyncio.get_event_loop().run_until_complete(some_test_fn()) == 2
+    _test_kwargs(some_test_fn, None)
+
+def test_decorator_default_sync_arg():
+    @a_sync.a_sync('sync')
+    async def some_test_fn() -> int:
+        return 2
+    with pytest.raises(TypeError):
+        asyncio.get_event_loop().run_until_complete(some_test_fn())
+    assert some_test_fn() == 2
+    _test_kwargs(some_test_fn, 'sync')
+
+def test_decorator_default_sync_kwarg():
+    @a_sync.a_sync(default='sync')
+    async def some_test_fn() -> int:
+        return 2
+    with pytest.raises(TypeError):
+        asyncio.get_event_loop().run_until_complete(some_test_fn())
+    assert some_test_fn() == 2
+    _test_kwargs(some_test_fn, 'sync')
+    
+def test_decorator_default_async_arg():
+    @a_sync.a_sync('async')
+    async def some_test_fn() -> int:
+        return 2
+    assert asyncio.get_event_loop().run_until_complete(some_test_fn()) == 2
+    _test_kwargs(some_test_fn, 'async')
+    
+def test_decorator_default_async_kwarg():
+    @a_sync.a_sync(default='async')
+    async def some_test_fn() -> int:
+        return 2
     assert asyncio.get_event_loop().run_until_complete(some_test_fn()) == 2
-    assert some_test_fn(sync=True) == 2
-    assert some_test_fn(asynchronous=False) == 2
+    _test_kwargs(some_test_fn, 'async')